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Les diagrammes suivants illustrent la mdthode. 1 2 3 1 2 3 4 5 6 Section HI., 1884. JRIGIN OF 30H.N f. ^y/ D.\ fi ••r ;.; -.i^LlNE ROCKS \ I ! : A. I J' I- " 1> A HISTORICAL AND CRITICAL REVIEW WITH AN AOOdUNT OK THE CRENITIC HYPOTHESIS /■ HV THOMAS STERRY HUNT MA. LLD,, P.H.S. FKO.M Till-; TRANSACTIONS OF THE ROYAL SOCIETY OF CANADA VOI^UME II SECTION III 1884 MONTREAL DAWSON BROTHERS PUBLISHERS 1884 •Joh a second uu ( \ The Library University ot Regina A f / \. Section III., 1884. [ 1 1 TuAN8. Roy. Soc. Canada. I. — The Ofifjin of CrysUdline Ruclcff. By Thomas Steriiy Hunt, M.A., LL.D. (Cantab.), F.R.S. (Presontod May 20, 1884.) L—IIisloriml and Oriticnl.—T\m .soliools of Woruor ami nf llnttoii. Tlio cliaotii', motainorpliic, motiwoiiiatic, tlii'.r- mocliaotic, ondoiiliitonic and oxopliitonii! or volcanic liyixjtliosps. Conditions of tlio problem. Tlio cro- nitic liyiHitlicsis stated. U.—Tlic Dtniopmcnt of a Nrw HypotlugiK.—TlM history of tiio growth of tlie crenitic liypotiio.sis. lll.—Illu.itratloiu of the Cremfic iri/pothens.—Tho hiatwy ot zoolitic and foldsi)atlii<; minerals; of the principal protoxyd-silicate.s, ami of other rock-formin;^ silicates. Tlio artificial proiluction of mineral silicates. Tlio conditions of tho crystalli/atioii of niinorahs. IV. — Coucluisimis ; followed by an analysis of the contents of t-octioiis, and a note. I. — Historical and Critical. ^ 1. The prol)lein of the origin of the crystalline rocks which rover so large a part of the earth's surface is justly regarded as one of fundamental importance to geology, and its solution has been attempted dixring the past century Ijy many investigators, who have advanced widely different hypotheses. These, it is i)roposed to review hrii'fly in a historical sketch before proceeding to suggest a new one, which it is the object of the present memoir to bring forward. Without going back to the speculations of the ancient philosophers, we find those of the last two centuries, Newton, Descartes, Leibnitz and Euffon, among others, accepting the hypothesis of a former igneous condition of our planet. Starting from this basis, the phenomena of volcanoes, and the reseml)lances between their consolidated lavas and many of the crystalline rocks, naturally gave rise to the notion of the igneous origin of these, which was formulated in the hypothesis that all such rocks, whether massive or schistose, were directly formed during the cooling and consolidation of a molten globe. § 2. Playfair, in his " Illustrations of the Huttoniau Theory of the Earth," tells us that it was Lehman, who, in 1*756, first distinguished by the name of Primitive the ancient crystalline rocks, described by him as arranged in beds, vertical or highly inclined in attitude, and overlaid by horizontal strata of secondary origin. These primitive rocks were by Lehman regarded " as parts of the original nucleiis of the globe, which had undergo'\c no alteration, but remained such as they were first created." This view was shared by Pallas and by De Luc, the latter of whom at one time considered the primitive rocks " as neither stratified nor formed by water," though as Playfiiir informs us, De Luc subsequently admitted " their formation from aqueous deposition, as the ueptunists do in general." ' ' John Playfair, loc. cit. pp. 100, 102. Tho Theory of tho Earth, by James ITutton, first ai)iioarod in 1785, and in a second edition iu 1795. Playfair's celebrated exposition of it, hero quoted, was published ia Edinburgh in 1802. Sec. III., 1884. 1, ^ 2 DE. THOMAS STHRRY HUNT ON THE Pallas hold a similar view, and according to Daiibreo, both Pallas and Sanssure " admitted, as Linnains had done, that all the terranos have been rormed by the agency of water, and that volcanic, phenomena are but local accidents." Pallas published his *' Observations on Mountains" in 1111, and Saussure the first volume of his " Voyages dans les Alpes" in 1119. It was about lISO that the celebrated professor of Freiberg began, in his lectures, the exposition of his views, called by Playlair " the lu'ptunian system as improved by Werner ;" though his Classification of the liocks, in which these views were finally embodied, dates only from 1*787. § 3. According to Werner, the materials which now form the solid crust of the globe were deposited from the waters of a primeval ocean, in which the elements of the crystalline rocks were at one time; diwsolved, and from which they were sei^arated as chemical precipitates. The granite, which he regarded as the fundamental rock, was first laid down, and was closely followed by the gneisses and the hornblendic and micaceous schists. When the dissolving ocean covered the whole globe to a great depth, and its waters were tranquil and pure, the rocks deposited were exclusively crystalline and, like the ocean, they were universal. These he distinguished as the Primitive rocks. At a later period, the depth of the ocean was supposed to have been diminished by the retreat of a portion of the waters to cavities within the globe ; a notion apparently borrowed from Leibnitz, who imagined caverns, left by the cooling of a formerly fused mass, to have snbsequejitly served as reservoirs for a part of the ixniversal ocean. In this second period, according to Werner, a chemical deposition of silicotes still went on, but dry laud having been exposed and shallows formed, currents destroyed portions of the previously deposited masses, which were also attacked by atmospheric agents. By these actions were formed mechanical sediments, which became interstratified with those of chemical origin. It was during thi.s period of co-incident chemical and mechanical deposition that were formed the Intermediate or Transition rocks of Werner, which, from the conditions of their formation, necessarily covered only portions of the universal Primitive series. At a still later period, marked by a farther diminution of the superficial waters, were laid down the Secondary rocks of Werner, at a time when the sea uo longer produced mineral silicates, and had assumed essentially its present composition. § 4. The Primitive rocks, according to this hypothesis, were those composed entirely of chemical deposits, which are either crystallized or have a tendency to crystallization, and in which the action of mechanical causes cannot be traced. In the Transition series, the products of chemical and mechanical processes are intermingled, and materials derived from the disintegration and decay of Primitive rocks are present ; while the rocks of the Secondary series were formed from the ruins alike of the Primitive and the Transition series. During the process of their consolidation, the various strata having been broken, fissures were formed through which the surplus waters retired to the internal cavities, depositing on the walls of the fissures through which they descended, the various matters still held in solution. In this way were formed metalliferous and other mineral veins. The aqueous solution in which all these crystalline rocks were at first dissolved was described by Werner and his disciples as a chaotic liquid, and he even designated the rocks themselves as chaotic, " because they were formed when the earth's surface was a chaos." These Primitive rocks, consisting of the granite and the overlying crystal- lino schists, covered the whole earth, and their geographical inequalities were due to ORIGIN OF CRYSTALLINE ROCKS. 8 the original deposition, which did not yield a regular surface, but presented elevations, upon the slopes of which were subsequently laid down the Transition strata. Such, according to Werner, was the origin of all rock-masses except recent alliavions, deposits of obviously organic origin, and the ejections of volcanoes, which he conceived to be due to the sul)terraneous combustion of carl)onacoous deposits. In the earlier ages of the world there were, according to him, no volcanoes and no evidences of subterranean heat. Neither in the formation of granite, of basalt, of the crystalline schists, or of mineral veins, or in the displacements of the strata to be seen in the deposits of various ages, did ho recognize any manifestations of an internal activity of the earth.- § 5. We now pass to the consideration of the rival geological theory of Hutton, which was developed at the same time with that of Werner. Saussure, as early as 1770, had ascribed to aqueous infiltration the granitic veins in the Valorsine, and others near Lyons — a view which was shared by Werner, who, from their similar constitution, conceived that the formation of massive and stratiform granitic rocks had taken place under conditions like those which gave rise to the veins in question, and then extended this view to other veins and masses of what we must regard as injected or irrupted rocks, including not only granites but dolerites and basalts. Hutton and his interpreter, Playfair, on the other hand, regarded all granitic veins as haA'ing been lilled by injection with matter in a state of igneous fusion, repudiating the notion of Saussure and of AYerner that such materials could be Ibrmed by crystallization from aqueous solutions. Grranitic veins, according to Hutton, are in all cases but ramifi- cations of great masses of granite, themselves often concealed from view. "In Hntton's theory, granite is regarded of more recent formation than the strata incumbent upon it ; as a substance which has been melted by heat, and which, forced up from the mineral regions, has elevated the strata at the same time." ' From this condition of igneous liquid- ity, he supposed, had crystallized alike quartz and feldspar, as well as tourmaline and the other minerals sometimes found in granitic veins. Granite is elsewhere declared by him to be matter fused in the central regions of the earth. § 6. With Werner, granite was the substratum underlying all other rocks simply because it had been the first deposit from the chaotic watery liquid, and it was said to pass into or to alternate with the distinctly stratiform or schistose crystalline rocks. In this view of its geognostical relations, Werner was strictly correct, if by granite we understand the massive or indistinctly stratiform aggregate which makes up what some would call granite and others fundamental granitoid gneiss. This is what I have called an INDIGENOUS rock, which may be witli or without apparent stratification. We must, how- ever, distinguish, besides this first type of crystalline rock, — the underlying granite of Werner, — two others which, though miuoralogically similar, and often confounded, are geognostically distinct. Of these, what I have called exotic rocks consist apparently of soft- ened and displaced portions of aggregates of the first type, and are met with alike in ' In preparing tho foregoing eynoiwis of tlio views of Werner, I have followed, in part, the exposition of his system given by Murray in his Review of Play fair's Illustrations of the Iluttonian Theory, iniblishod anonymously in Edinburgh in 1802 ; in part tho statements to be found in Playfair, in Bakowoll, in Lyoll, and in Naumann ; and also the excellent analysis given by Daubr& in his Etudes ot Exi)drieiicos Synth6tiquos sur le M6tamorphismo, et sur la Formation dcs Roches Cristallines ; Paris, 1860. ' Playfair, Illustrations, etc, p. 89. DR. THOMAS STERRY HUNT ON THE dykes and in masses of greater or less size, intruded or irrupted among the stratified or indigenous rocks. These are the typical granites of Hutton. The third type includes those concretionary masses of granitic material formed in fissures or cavities, which are evidently deposits from aqueous solutions. These are the infiltrated veins of Saussure and of Werner, and are Avhat I have designated endooenous rocks. § *7. By keeping in view this threefold distinction between indigenous, exotic and endogenous granitic aggregates, as I have long since endeavored to show, the obscurities and apparently contradictory views of dific'rent observers are easily exjilained. These distinctions are recognized in other crvstallinii rocks than granite. Under the name of crystalline limestones, as is well known, have been included both indigenous and endoge- nous masses. The question whether or not certain crystalline silicated rocks are to be regarded as eruptive, is seen to be of minor importance,when we consider that it is possible for indigenous crystalline deposits to appear in the relation of exotic masses ; whether dis- placed in a softened and plastic condition, as more generally happens, or else forced, in rigid masses, among softer and more yielding strata, as appears, from the observations of StapfF, to be the case of the serpentines of St. Gothard. ' § 8. Werner argued, and as we shall endeavor to show, correctly, from their analogies with concretionary granitic veins, that all granitic; rocks were deposited from water, and are consequently indigenous or endogenous in origin. He denied the existence of exotic and of igneous rocks. Hutton, on the contrary, from the phenomena of exotic granites, and the analogies observed between these and basalts and modern volcanic rocks, was led to assiime an igneous and exotic origin for all save th(! clearly stratiform crystalline rocks. Metalliferous lodes, also, he supposed to have been formed, like granitic veins, by igneous injection from below. AVhile the disciples of Werner denied the igneous origin of basalts, and even of obsidian, Hutton and his school, on the other hand, maintained that the agates often found in erupted rocks were formed by fire. Playfair reasons : — " The lluidity of the agate was therefore simple and unassisted by any menstruum ;" that is, it was due to heat, and not to solution ; while, in the case of mineral veins, their closed cavities were held to " allbrd a demonstration that no chemical solvent was ever included in them." '' These cavities were regarded as due to the contraction consequent on the cooling of injected igneous material. § 9. The basic rocks, included l)y Hutton under the common names of basalt and whin- stone, are regarded by him as similar in origin to granite, and called " iinerupted lavas." He elsewhere says that " whinstone is neither of A'olcanic nor of ac^ueous, but certainly of igneous origin," that is to say plutonic. Playfair distingiiishes between what ho calls the volcanic and the plutonic theory of basalt. But while Hutton ascribed a plutonic origin to basalt and to granite, he did not, as some have done, assign a similar plutonic. origin to gneiss and other crystalline schists. These were by Werner declared to result from a continuation of the same process which gave rise to granite, and to graduate into it. Gneiss is held ])oth by Wernerians and by modern plutonisls to be but a stratiform granite, and both of these rocks are believed by the one school to be aqueous and by the other to bo igneous in origin. * Soe Trans. Royal Sop. Canada, vol. i, part iv, pago 212. ' Playfair, Illustrations, etc., pp. 79 and 260. r ORIGIN OP CRYSTALLINE ROCKS. In the system of Hutton, however, a wide distinction is made between the two rocks. Gneiss was no longer a primitive or original rock, as taixght by Lehman and by Werner, bnt, like the other crystalline schists, designated by Ilutton as Primary, was supposed to be " formed of materials deposited at the bottom of the sea, and collected from th<^ waste of rocks still more ancient." In his system " water is first employed to arrange, and then lire to consolidate, mineralize, and histly to elevate the strata; but with respect to the unstra- tilled or crystallized substances the action of fire alone is recognized." '' Hutton also con- ceived the pressure of the waters of a superincumbent ocean to exert an important influ- ence in the consolidation of the sediments. He is thus a plutonist only so far as regards granite and other unstratili' I rocks, while in maintaining a detrital origin for the crystal- line schists he, as Naumann has remarked, may be regarded as the author of the so-called metamorphic hypothesis of their origin. Playfair himself declares of Hutton's system : "We are to consider this theory as hardly less distinguished from the hypothesis of the vulcanists, in the xxsual sense of this appellation, than it is from that of the neptunists or distiiples of Werner." " § 10. It was no part of Hutton's plan to discuss the origin of those more ancient rocks, which had, according to him, furnished by their disintegration materials for the primary stratified rocks. It was, in the language of Playfair, a system " where nothing is to be seen beyond the continuation of the present order." " His object was not . . . like that of most other theorists to explain the first origin of things." This system, as interpreted by his school, asserts the conversion of detrital rocks into masses indistinguishable from those of truly igneous origin, which were the sources of the first detritus. The changes whi<-h it assumed to be wrought by the alternate action of water and fire on the earth's crust were not supposed to lie limited by any external conditions in the nature of things, and were compared by Playfair to the self-limited perturbations in the movements of the heavenly bodies, in Avhich, as in the geological changes of the earth's crust, " we discern no mark either of the commencement or termination of the present order." § 11. Hutton's system is thus concisely r<>sumed by Daul)rce ; — " The atmosphere is the region in which the rocks decay ; their ruins accixmulat(! in the ocean, and are tliere mine- ralized and transformed, ixnder the double influence of pressure and the internal heat, into crystalline rocks having the aspect of the older ones. These re-formed rocks are subse- quently ixplifted by the same internal heat, and destroyed in their turn. Th(> disintegration of one part of the globe thus serves constantly for the reconstruction of other parts, and the continued absorption of the underlying deposits produces 'incessantly new molten rocks, which may be injected among the overlyhig sediments. We have thus a system of destruction and renovation of which we can discern neither the beginning nor the end."' § 12. It was this perpetual round of geological changes, which took no account either of a beginning or an end, that led the theologians of his day to oppose the system of Ilutton. On the other hand, in the system of Werner, which taught the fashioning of the present order of our globe from a primeval chaos beneath the waters of a universal ocean, they saw a conformity with the Hebrew cosmogony which recommended to them the neptunian " Playfair, Ilhistrationa, otc, pp. 12 and 131. ' Biography of Hutton ; I'layfair's Worlds, vol. iv., p. 52. " Daul)r6o, l^ltudos et ExixSrionces, etc., p. 12. 6 DR. THOMAS STERRY HUNT ON THE hypothesis. Hence the theological element which, as is well known, entered so largely into the controversies of the vulcanists and the noptunists at the beginning of this century, and the suspicion with which the partisans of Hutton were then regarded by the Christian world. The extreme neptunian views of Werner, however, soon fell into disfavor. The visible evidences of the cxtrnsion of trappean rocks in a heated and softened state, obser- vations showing the augmentation of the temperature in mines, a. the phenomena of thermal springs and volianoes, .soon turiu'd the scale in favor of llutton's views. There were not wanting tho,se who attempted to unite the Wernerian hypothesis with that of an igneous globe, and who supposed a primeval chaotic ocean, to the waters of which, heated by the mass below, and kept at a high boiling-point by the pressure of an atmosphere of great density, was ascribed an exalted solvent power. § 13. 8uch a modilied neptunian view was advanced by Delabeche. In his "Researches in Theoretical Crcoiogy," published in 18;]'7, he favored the notion of an unoxydized nucleus, as suggested by Davy, and held to a solid crust resting on a liquid interior, and presenting from the first, irregularities of surface. He then speaks of " the much debated question " whether the crystalline stratified rocks " have resulted from the deposit of abraded por- tions of pre-existing roiks mechanically suspended in water, or have been chemically derived from an aqueous or an igneoiis fluid in which their elements were disseminated." We have in this paragraph three distinct hypotheses presented. Two years later he clearly declared for the second of them. Whil<' admitting the crystallization of detrital matter in proximity to intrusive rocks. Delabeche? obji'ctcd to what he called the " swc^eping hypotliesis " of Hutton and his school, He supposed that, in the cooling of our planet from an igneous fluid state, "there must have been a time when solid rock was first formed, and also a time when heated fluids rested upon it. The latter would be conditions highly favorable to the production of crys- talline substances, and the state of the earth's surface would then bo so totally different from that which now exists, that mineral matter, even when abraded from any part of the earth's crust which may have ])een solid, would be placed under very different conditions at these different periods." He suggests that there would bo " a mass of crystalline rocks produced at first, which, however they may vary in minor points, should still preserve a general character and aspect, the result of the first changes of fluid into solid matter, crystalline and sub-cry.stalline substances prevailing, interminghid with detrital portions of the same substances abraded by the movements of the heated and first-formed aqueous fluids. In the gneiss, mica-slate, chloritic-slate and other rocks of the same kind, associated together in great masses, and covering large areas in various parts of the world, we seem to have those mineral bodies which were first formed. The theory of a cooling globe, such as our planet, supposes a transition from a state of things highly favorable to the production of (Crystalline rocks, to one in which masses of these rocks would be more rarely formed. Hence we could never expect to draw fine lines of demarcation between the products of one state of things and those of the other." ^ § 14. Still later, in 18G0, we find a similar view svxggested by Daubree as a probable hypothesis. He goes back in imagination to a time when the waters of our planet, as yet ' Delabecho, Geology of Cornwall and Devon, pp. 33-34; also Resoarclioa in Theoretical Geology. ORIGIN OF CRYSTALLITE? ROCKS. uncondensed, surrounded the globe with a dense envelope estimated to equal 250 atmos- pheres. " The surface of the earth was at this time at a very high teraporatun>, and if silicates then existed they must have l)eeu formed without the co-.operatiou of liquid water. Later, however, when it began to assume a liquid state, the water must have reacted upon the pre-existing silicates ui)ou which it reposed, and then have given risi; to a whole series of new products. My a veritable naetamorphic action, tht; water of this primi- tive Goean, penetrating the igneous masses, caused their primitive characters to disappear, and formed, as in our tubes, crystallized minerals from the matters which it was able to dissolve. These matters, formed or suspended in the liquid, would then be precipitated, and give rise to deposits presenting dillerent characters as the temperature of the liquid diminished." lie then inquires: "Were these dillerent periods of chemical decompo.sition and recomposition, in which aqueous action {la vote humule) intervenes under extreme con- ditions which approach those of igneous action {In vote sdrhe), the era of the formation of granite and of the azoic and crystalline sschisls? We cannot adirm this in an absolute manner, but we may presume it, especially when we consider that on this hypothesis there must have been formed two principal products, the one massive and the other presenting evidences of sedimentation, passing into each other gradually, as is the case with granite and gneiss. In any case, it cannot be contested that if there was a time when the rocks were exclusively under the dominion of fire, they passed under that of Avater at an epoch much more remote than we have hitherto admitted. The inllucnce, now established, of water in the crystallization of silicates, no longer permits any doubt on this point. AVe cannot perhaps now find anywhere upon the globe rocks of which it may be affirmed \vi( h certainty that they have been formed by igneous action, without the intervention ol water." '" § 15. To give some notion of the temperature of the first water precipitated on the earth's cooling surface, Daubree calculates that the waters of the present ocean, estimating their mean depth at 3,500 metres, would, if spread uniformly over the earth's surface, have a thickness of 2,56-5 metres, which, if converted into vapor, would correspond to a pressure of 248 atmospheres, a weight which would be augmented by the presence of other vapors and gases. " No liquid water could therefore rest upon the earth until its temperature had fallen below that which would give to the vapor of water a tension of 250 atmos- pheres," at least. When we consider that a tension of only fifty atmospheres of steam corresponds, according to Arago and Dulong, to a temperature of 2()5'89 centigrade, we can form some conception of that corresponding to a tension five times as great ; which would, on this hypothesis, have been the temperature of the first waters precipitated on the cool- ing planet, realizing many of the conditions attained by this ingenious experimenter when he subjected mineral silicates to the action of water in tubes, at temperatvires of from 400' to 500' centigrade. It is unnecessary to point out that Daubree here attempts to adapt Werner's neptunian hypothesis to that of a once-fused and cooling globe, and to find, like Delabeche, in the highly-heated primeval ocean, the chaoticliquid which, according to the master of Freiberg, was the menstruum which at one time held in solution the elements of the primitive rocks. The experiments of Daub.ee in his tubes, above referred to, are of great impor- '°Daubr6e, Etudes et Exp^iioiicos Syuthotiiiuos, etc., pp. 121, 122, 8 DR. TnOMAS STEIiKY HUNT ON TlUi tance in this counectiou, and will bo considered farther on, in the third part of this paper. § 10. The Huttouians early borrowed the notion of a granitic substratum from Werner, a;.'>l supposed the earth when first cooled to have had a su'-face of granite. Hutton, true to his thesis, avoided the question of the primal rock. His reasonings, according to I'layfuir, "leave no doubt that the strata which now compose our continents are all formed from strata more ancient than themselves," " while, as we have seen, the intruded granites were looked upon as but fused and displaced portions of underlying strata. The granitic character of the rocks which antedated aqueous disintegration was, however, a matter of legitimate inference, and his disciple, MaccuUoch, supposed the earth when first cooled to have been " a globe of granite." Later, in 1847, Elie de Beaumont, starting from the hypothesis of a cooling liquid globe, imagined it " a ball of molten matter, on the surface of which the iirst granites crystallized." '" § 17. It should here be mentioned that Poulett Scrope, in 1825, put forth what he called "A New Theory of the Earth," in which he supposes " the mass of the globe, or at least its external zone to a considerable depth, to have been originally (that is at or before the moment in which it ...sumed the position it now holds in the planetary system) of a granitic composition, composed probably of the ordinary elements of granite, and having a very large grain ; the regidar crystallizat^ion having been favored by the circumstances under which it previously took place, though, as to what these circumstances were, I do not venture to hazaid a supposition." He farther savs. "If then we imagine a general intumescence of an intensely heated bed of granite, forming the original surface of the globe, to have been succeeded by a period in Avhich the predominance was acquired by the repressive force occasioned l)y the condensation of the waters on its surface, and the depo- sition from them of various arenaceous and sodimental strata (the transition series), the structure of the gneiss-formation is at once simply explained. This structure may have been subsequently increased by the friction of the dillerent lamin;e against one another as they were urged forward in the direction of their plane surfaces, towards the orifice of protrusion, along the expanding granite beneath ; the lamina3 being elongated, and the crystals forced to arrange themselves in the direction of the movement." This implies an exoplutonic origin /»f gneiss. Later in the same essay, however, Scrope supposes an intensely heated ocean, holding in solution great amounts of silica, and having, at the same time, suspended in its waters, feldspar, quartz and mica, derived from the disintegration of the underlying granite. These suspended materials were deposited and consolidated into gneiss, and later the dissolved silica, precipitating with some enclosed mica as the ocean cooled, gave rise to mica-schists. In this last, we see the germ of the thermochaotic hypothesis, while in preceding statements of Scrope, we have outlined the early volcanic and metamorphic hypothesis of Dana, to be noticed farther on." " I'layfair's liiography of James Hutton, in Playfair's comploto works, 4 vols, Eilinburgh, 1822 ; soo vol. iv, pp. 33-8i. Kis nil strations of tho Iluttonian Tlioory will tlioro Iw foim' of the Fortiotli Parallel, vol. I, p. 117. ORIGIN OF CRYSTALLINE ROCKS. IS heated waters, at great depths in the earth. ^' All this is but a repetition ol' the hypo- thesis put forward forty years since by Dana, and subsequently abandoned by him. Tornebohm has also lately advanced a similar hypothesis to explain the origin ol' the primitive granite, and of the gneiss into which it seems to graduate. The material of these rocks came up as lava now does, and a portion of it, disintegrated, rearranged by water, and recrystallized, assumed the form of gneiss, lleusch, in like manner, according to Marr, supposes that the gabbros, diorites, and dioritic and hornblondic-schists of the Bergen district, in Norway, are but altered tufas and erupted rocks. § 31. Mr. Marr, in a recent paper, urges the claims of the volcanic hypothesis to explain the origin of the ancient crystalline rocks, seemingly unaware of its earlier advocates. It is apparent that if we accept the doctrine of the permanence of continents and of oceanic depressions, the metamorphic-detrital theory of the Huttonians, which builds up series of crystalline rocks beneath the sea from the ruins of an older land, which had itself been formed beneath the sea, is no longer tenable. The difficulty of getting the thirty thousand feet of sediments required to spread over a continent, as in Dana's later hypothesis, is, as Marr perceives, overcome if we suppose this material to have been derived not by the superficial waste and disintegration of former land, but by ejection from reservoirs beneath the earth's crust. Hence, with the advocates of the doctrine of the permanence of con- tinents, the volcanic or exoplutonic hypothesis is again coming into favor. -' Similar considerations appear to have led C. 11. Hitchcock, in 1883, to a like conclusion. The continents, in his scheme, are built up from beneath the waters of a universal oiiean. He writes : — " "We start with the earth in the condition of igneous lluidity. It (;ools so as to become encrusted and covered with an ocean. Numerous volcanoes discharge molten rock, building up ovoidal piles of granite [beneath the ocean], which change gradually into crystalline schists. When the hills are high enough to overlook the water, they constitute the beginnings of dry land." All this is intelligible, but it seems strange to one familiar with the geological literature of the last forty years to read, in this connection, the remark that " few have ventured to speak of anything like volcanic action, except as it has been manifested in the formation of dykes, in the early periods." -' To all of these speculations as to the exoplutonic or volcanic origin of the crystalline rocks, the language of Naumann, in criticizing the original volcanic hypothesis of Dana, is applicable. " The perfect and thoroughly crystalline character of the gneiss, the enormous extent which the primitive formations occupy in so many districts, the architecture of these great gneissic regions, and their occurrence wholly independent of larger granitic masses, are all incompatible with this idea." § 32. The view of the igneous and eruptive origin of crystalline limestone, admitted in Dana's former scheme, was familiar to the geologists of forty years since. Emmons and Mather in America, and von Leonhard, Rozet and Savi in Europe, among others, then held to the belief that many crystalline limestones were igneous, and Savi had even attempted to point out the centres of eruption of the Carrara marbles.'^' It is hardly neces- '" Noues Jahrbuch fur Minoralogio, 1872, pp. 388 aiul 4i)0. '" Marr, Tlio Origin of ArchsBan Rwks ; Geological Magazine, Juno, 1883. " Hitchcock, The Early History of tlio North American Continent.— I'roc. Amor. Assoc. Adv. Science, 1883. ""See for references Hunt, Chem. and Geol, Essays, p. 218; also Bouc, Guide du Gtjologuo Voyageur, ii. 108, 16 DR. THOMAS STERRY HUNT ON THE sary to recall the fact that serpentines, and great deposits of magnetite and specular iron, are istill by some authorities considered as eruptive rocks, and that the hypothesis of the igneous origin of metalliferous lodes, taught by Ilutton, is not yet wholly obsolete. In 1858, H. D. Rogers spoke of '• the great dykes and veins of auriferous quartz " supposed to have issued " in a melted condition through rents and fissures in the earth's crust. Out- gushing bodies of this quartz," chilled by contact vfith the cold waters of the ocean, were supposed by him to have furnished the material for the Primal quartzites of Pennsylva- nia. " Still later, in 18*74, we find Belt maintaining with learned ingenuity the igneous origin and the injection of auriferous quartz veins. He insists, as I have elsewhere done, ^ on the transition from veins of pure quartz, often metalliferous, to others containing feld- spar, and thence to true granitic veins ; but instead of regarding these as aqueous and concretionary, assumes them to be igneous, and thence concludes that the gold-bearing quartz lodes were filled with liquid quartz by " igneous injection," though admitting that in these, as in granites, water helped to impart liquidity. -'" § 33. In farther illustration of the extiMision of the plutonic doctrine to other rock- masses than those already mentioned, I quote from an essay by Daubree, published in ISTl.*" "The hypothesis advanced by Lazzaro Moro, in 1Y40, attributing an eruptive origin to rock- salt as well as to sulphur and bitumen, was again taken up and applied by de Charpentier (1823) to the salt-mass at Box, which is associated with anhydrite ; and d'Alberti, in the classic study made by him of this terrane, maintained tlie same hypothesis for all the rock- salt found in the trias. Moreover, the examination of the deposits of pisolitic iron ore had, in 1828, conducted Alexandre Brongniart to a similar conclusion, which was soon after applied to the silicious deposits which constitute the buhrstone of the tertiary. A like origin was by d'Omalius (1841 and 1855) ascribed to other substances, particularly to certain clays and to certain sands, which, especially in Belgium, appear to be connected with the formation of calamine, and which Dumont in 1854 called geyserian deposits." " It was thus," adds Daubree, " that various substances belonging to sedimentary strata were recognized as coming, or at least were supposed to come, from the lower regions (elaient recommes ou nu moins etaient supposces provenir des regions profondes.)" § 84. The presence of water in ignited and molten rocks was shown by Poulett Scrope in 1825 in his studies of volcanoes.^' Subsequently, Scheerer, conceived that a small portion of water, probably five or ten hundredths, might, at a low red heat, give rise to a condition of imperfect liquidity such as he imagined for the material of eruptive granites. Similar ideas as to the aqueo-igneous fusion of granite were at the same time adopted by Elie de Beaumont, and are no-> generally admitted, the more so, as they are in accordance with the results of microscopic study. From the presence in granitic rocks of what he called pyrognomic minerals, like allanite and gadolinite which, by exposure to igiiition> " Geology of Pennsylvania. II. 780. ** Chemical and Geological Essays, pp. 192-208, and infra Part II. "Belt, The Naturalist in Nicaragua, 1874, pp. 97-100. In tlie pages here referred to, my friend, whose premature death was a great loss to science, has set forth with clearness the Huttonian theory of niotalliforous veins. "Daubr^, Des terrains stratifies considfirt's au point de vue de I'origino des substances qui les constituent, etc. Bull. Soc. G^ol. de France (2) xxviii, p. 807. " Scrope, Considerations on Volcanoes, p. 25- ORIGIN OP ORYSTALLTNF'^ ROOKS. 17 undergo porraauoiit physical and chemical changes, Schoerer, moreover, argued that the temperature of formation of the granitic veins holding these minerals could not have ])een very high. ^'■ This notion of hydroplntonio eruptions, thus set forth by Srrope, Scheerer and Elie de Beaumont, has received a still farther extension of late. The liydrated rock, serpentine, is supposed by some of those who maintain its exoplutonic derivation to have come up from below as an anhydrous silicate, and to have been subsequently hydrated. Daubree, how- ever, has suggested that it had already pas.sed into the hydrated condition before its ejec- tion." Akin to this is the view of some modern Italian geologists, who explain the strati- form character of this roi^k by supposing that it was ejected from below as an atiueous magma, chiefly of hydrated silicates of magnesia and iron mingled in some cases with feld.s- pathic matter ; from which, by crystallization and rearrangement, the masses of serpentine and their associated euphotides have been formi'd, as well as the accompanying anhydrous silicates, olivine and enstatite. Ey this hypothesis "the serpentines are considered as eruptive without being truly igneous, inasmuch as they do not contain in their composi- tion any mineral which has been submitted to igneous fusion," though " the magma may have had a temperature of several hundred degrees." " The conception of hydroph\tonic eruptions, whether applied by Scrope to lavas, by Scheerer to granites, by Belt to metalliferous quartz lodes, or ])y Daubree and some Italian geologists to serpentines and euphotides, is instructive as a phase in the development of that geologii-al hypothesis, according to which a volcano is a deua ex machimi, which is invoked to solve every knotty i>roblem that presents itself in sti;dying the origin of rock-masses. § 35. Writing in 1883 of the extravagances of the exoplutonic or volcanic doctrine, I spoke of it as " the belief in a subterranean providence which coixld send forth at will from its reservoirs " alike granite and basalt, olivine-rock and limestone, quartz-roik and magnetite. •''■'' An otherwise friendly critic'"' speaks of this language as " a kind of device for producing a false impression, by associating rocks for the most part of eruptive origin with others which are not so. ' This, however, is precisely what the plu )uic school in question has done, and is still doing. Eminent teachers in geology of our time, some of them still living, have included with granites and basalts, not only serpentines, but lime- stones, magnetite, auriferous quartz, buhrstone, rock-salt, anhydrite, hydrous iron-ores, and even certain clays and sands, among the substances which have been thrown up from the depths of the earth. The obvious question, as to the origin of these supposed accximulations of various and unlike substances in the under- world, has been one to perplex the thoughtful geolo- gists of this school, and for those who did not admit that such might ('ome from buried deposits, once superficial, presented difficulties which it was sought to overcome by a ''' For an analysis of tlioso views of Schooror and ftlio do Beaumont, ami roforonces to tlio eontrovorsios to which thoy gave rise, see Hunt, Chemical and Geological Essays, pp. 5, 0, and ISS, 189. '^ Geologic Experimontale, p. 542. "* See, for an account of this hypothesis as maintained by Issol and Capacci, Hunt, on The Geological History of Serpentines, Trans. Roy. Soc. Can., vol. i., part iv., p. 198. ^ Ibid, vol. i., part iv., p. 206. ^ Geological Magazine for June, 1884, page 278. Sec. ILL, 1884. 3. 18 DR. THOMAS STER]IY HUNT ON THK general theory of transmutation ; by which it was imagined that n part or the whole of the original elements of a rock might be replaced, thus giving rise to new lithological spe- cies. Such a chimge has been appropriately named a metasomatosis or change of body. I have elsewhere pointed out that this view has been adopted by two distinct and, to a certain extent, opposed schools in geology, both of which, however, agree in admitting an almost unlimited capacity of change of substance, through aqueous agencies, in previously solidilied rocks. Tlie first of these schools applies the doctrine of metasomatosis to silicated and aluminous rocks, either of plutonic or plutonic-detrital origin ; the second to rocks of generally acknowledged aqueous origin, such as limestones.'' § 36. As regards the metasomatosis of plutonic or plutonic-detrital rocks, such as the ordinary feldspathic types, — granites, gneisses, diaba,ses and diorites, — we are taught the conversion of any one or all of these into serpentine or into limestone. The uitegral change of each one of these into serpentine by the complete elimination of alumina, alkalies and lime, and the replacement of these bases by magnesia and water has, as is well known, been maintained by many writers of repute, including Miiller and Bischof, and later Dana and Delesse. Moreover, King and Rowney have, since 18'74, taught the conversion into limestones of all the silicated rocks mentioned, and have assigned a similar origin to the great interstralilied masses of crystalline limestone which are found in the ancient gneis- ses, alike of North America and Europe. Not content with this, they have even maintained the conversion of serpentine itself into limestone, and have explained the existence of ophicalcites, and of serpentine masses in limestone, as evidences of the incomplete trans- formation of beds of serpentine, itself the product of a previous transformation of feldspathic rocks. ^^ The older school of metasomatists regarded serpentine and other hydrated magnesian silicates, on account of their insolubility, iis the last term in the metasomatic process ; but King and Rowuey contend that serpentine itself is not exempt from change. § 3Y. Among the gneisses and mica-schists of the Atlantic belt are found at many points, especially in Pennsylvania and thence southwestward throiigh the Carolinas into Alabama, important masses of a rock composed essentially of a chrysolite or olivine, and referred to dunite or Iherzolite. With these are associated not only serpentine but various hornblendic and feldspathic rocks, together with much corundum — the latter alike in segregated veins and disseminated in the beds. These c;hrysolite-rocks, which, as seen in North Carolina, were already described by the writer, in 1879, as indigenous stratified deposits in the Montalban series," have been made the subject of detailed studies both by G-enth and by Julien, whose published results are instructive examples of the application of the metasomatic doctrine in the hands of its disciples. Genth supposes that, at the time when these chrysolite-rocks were deposited, vast amounts of alumina were set free by some unexplained process, and formed beds of corundum, and that this species, by Bub- ^' See, in this connection, Hunt, Cliem. and Geo!. Essays, pp. 31G, 320, 325 ; also preface to the second edition of the same, pp. xxvii-xxxi.; and fartlior, Trans. Roy. Soc.Can., vol. i., part 4, pp. 168-204. ""Cliem. and Geol. Es.says, p. 324; also Trans. Roy. Soc. Can., I., part 4, p. 204; and W. King and T. H. Rownoy, An Old Chapter of the Geological Record, 1881, chaiw. vii. and sii. *• See Jamea Macfarlane's Geological Handbook, 1879, p. 130 ; and, for some notes on the history of similar rocks, Tran. Roy. Soc Can., vol. i., part 4, p. 210. ORIGIN OF CRYSTALLINK ROCKS. 19 sequent hydration and metasomatosis, has been changed to bauxito, diasporo, spinel, opal, and a great number of aluminilerous silirates, including various mieas, probably some feldspars, and also magnesian silicates of the chloritic group. Tho iinal result has been, " in many instances, a pretty thorough alteration of the original ("orundum into micaceous and chloritic schists or beds, or, as Prof. Dana would express it, ' a pseudomorphism on a broad scale.' " "' § 38. Julien, who has more recently studied these rocks, adopts with regard to the chrysolite-beds the view suggested by Clarence King, in IHTH, that they wt^re derived from the disintegration of chrysolitic eruptive rocks, and were originally chrysolite sand- stones. Chrysolite, according to him, and not coruiulum, has been the point of departure for the various changes which have given rise to the crystalline schists in question. Thus, while some of the chrysolite beds remain unchanged, others luive been converted into strata of cellular ihalcedonic quartz, of serpentine, of steatite, of talcose actinolite-schist, of tremolite schist, and of a diorite or gabbro made of albite and smaragdite and including grains of red corundum, sometimes with margarite. AVithin these rocks are veins and lissures of various sizes and shapes, in which are found crystallized corundum, with ensta- tite, actinolite, talc and ripidolite, among o'hcr species. Julien, who assigns a similar origin to the like crystalline schists found elsewhere throughout the Atlantic belt, con- cludes that all of these various rocks have been derived from chrysolite. As regards the hypothesis of Genth, he writes : " The view which has been suggested, founded on certain phenomena observed in the corundum-veins, that these sec^ondary rocks, and many schists, have been derived from the alteration of corundum, finds not the least confirmation from my studies, and is indeed strongly contradicted by facts observed in the field. The corun- dum itself is, in all cases, both in the veins and in the particles found in the gabbro, a secondary or alteration-product. All the phenomena of alteration, both in the veins and rock-masses absolutely require, and can be simply explained by the introduction of a solu- tion of soda and alumina into the fissures and interstices, during the period of alteration and metamorphism." ^' This solution, he imagines to have come from some subterranean source *"Genth, Proc. Amor. I'liilos. Soe., Stipt. 1873 and July 1874; also Amor. Jour. Sci. (3), vi., 4(il and viii., 221- 223. Mr. Dana, in a notice of Dr. Gontli's conclusiona, in tim last citation, donouncos mo sovondy fur liavinj,', on a former occasion, cited from him tiio words above quoted Ijy Gontli, for^ottin},' tliat it is Gentli, wlioni ho praises, and not myself, who is thus attributing thorn to liim, and that Gonth's conclusions, if admitted, form a striking oxoniplification of that doctrine, which Dana thoro repudiates. In the same note, after stating tliat I have declared that "the advocaU>a of tho doctrine of transnnitation" have tauglit that "the greater part of all tho so-callod motamorphic or crystalline rocks are tho result of an epigenic process," and tliat "tho advocates of this doctrine maintain that a mas.s of granite or diorit« may l^e converted into serpentine or limestone, and tliat a lime- stone may bo changivl into granite or gneiss, which may in its turn become serpentine," Dana calls this an extra- vagant doctrine, and says:—" I demonstrattxl that all writers on p.seudoniorpliism, with but one or two exceptions, would repudiate it as strongly as myself." lie farther says the stateiuonts here quoted " liavo Iwen shown by me to bo untrue;" and, willi regard to the transmutation of granite or gneiss into limcstono, declares, in repeat- ing his charges before the Boston Society of Natural History, that " he never knew any ono ignorant enough or audacious enough to have suggested it" (Proc. Boston Soc. Nat. Hist., xvii. p. 170.) Those who read tliese pagea, and will take tho trouble to consult tho authorities here cited, or those given in more detail in my Chemical and Geological Essays, pp. 324-320, may satisfy themselves that I have not borne fatso witness in this matter, but that every one of tho changes cited has been formally maintained l>y some ono or more of the transmutatioiiists. It is surely not more difficult to transform granite into limestone, than limestone to granite, as imagined by Volger, or corundum to opal with Genth, or chrysolite to corundum with Julien. *' Proc. Boston Soc. Nat. Hist (1883) vol. xxiii, p. 147. 20 DB. THOMAS STRRRY HUNT ON TTTR in a hoatod condition. Tho applications of tho doctrine of metasomatosis seem to l)o limited only by the iniaginatioii of its disciples. § 89. Wo now corao to examine what we have called th(^ second phase of the doctrine of metasomatism, whiasoning adopted by this school. The occurrence of feldspar in the form of cal- cite, according to him, "proves the possibility of carbonate of lime being replaced by a feldspathic substance." IIo elsewhere argues that since both quartz and feldspar may repla< c calcite, " if both changes take place together, the chief constituents of gneiss would be substituted for the limestone removed." " Volger also describes instances of the asso- ciation ol adialaria and pericline with calcite, at St. Gothard, which show that feldspar, quartz and mica may be substituted for the carbonate of lime in calcite. Consequently, it may be inferred that granite or giuuss may be produced from limestone in the same manner." '- § 40. Akin to this view of Volger is that suggested by Pumpelly with regard to the halleflinta or bedded petrosilex-porphyry of Missouri (composed chietly of quartz and orthodase) — that this rock, as well as its imbedded magnetic' and specular iron and man- ganese ores, may have been deriA^^d by a nietasomalic process from a limestone, parts of which were replaced by the oxyds of iron and manganese, " while the porphyry, now sur- roiiiiding the ores, may be due to a previous, contemporaneous, or subsequent replacement of the lime-carbonate by silica and silicates." Portions of this pelrosilex are, in fact, inti- mately mingled with calcite, and thin layers of crystalline limestone are also found inter- slralilied with tlie petrosilex, which, in these associations, retains its normal composition of a mixture of orthodase and quartz " The hypothesis of metasomatism as applied to silicated rocks, endeavors to account for the generation of diil'erent and unlike masses in a single crystalline terrane or series, and iilso for certain phenomena in the transformation of detrital rocks. As applied to lime- stones, however, by Eose, Volger, Bis(;hof and Tumpelly, it seeks to explain the transforma- tion of a single wide-spread rock into granite, gneiss, serpentine, petrosilex, and crystalline iron-ores. These transformations once established, we should have an intelligible hypothesis to account for the origin of the principal crystalline rocks. § 41. We have in thi; preceding historical sketch endeavoured to shew that the existing hypotheses regarding the origin of the stratiform crystalline rocks may be dassed under six heads, which are as follows: — « Bischof ; Cliomical and Pliysical Goolo^'y, 1859, vol. IIT., pp. -531, 432. " Goologiual Survey of Missouri, 1873; Iron Ore*, etc., pp. 2i'>27. Also Hunt, Azoic Rocks, Ron. K, Second Geological Survey of Pcnn., p. li)4. ORIGIN OF CRYSTALLTNI-: ROCKS. 21 I. Endoplutonio. This snpposoH the rocks in quoHtion to havo heon formotl from tho mass of tha primeval glol)o as it oongoal(>(l from iijneouM fuNi(m iinhcock). The heat, which was believed to effect the metamorphosis of these detrital materials beneath the sea into crystalline rocks, is supposed by the Huttonians to have come from the heated interior by conduction, but, according to the volcanic-detrital hypothesis of Dana, through the direct heating of the waters of the sea by contact with the eruptive matters. IV. Metasomatio. Although the crystalline rocks believed to be formed in each one of the preceding methods have been supposed to be occasionally the subject of wide- spread metasomatosis, we may properly restrict the title of a general raetasomatic hypo- thesis to that which seeks to explain the derivation of the principal crystalline silicated rocks from limestones, as suggested by Hose, Yolger, Bischof and Pumpellj'. V. Chaotic. We have already suggested the name of the chaotic hypothesis for that which supposes the crystalline stratiform rocks, as well as the granites landerlying them, to have been successively deposited by crystallization from a general chaotic ocean, by which their elements were originally held in solution. In this doctrine, which was taught by Werner and his immediate disciples, the conception of internal heat was not recognized, and there was no suggestion of an elevated temperature in the chaotic ocean. VI. Theumociiaotic. The history of the attempts to adapt the Wernerian hypo- thesis to the conception of a cooling globe has already been told in the preceding pages. It was supposed that the waters of the universal chaotic ocean were highly heated, and were thus enabled to exert a powerful solvent action upon the previously-formed plutonic rocks of the primitive crust, transforming them into the present crystalline stratiform 22 DR. THOMAS STERRY HUNT ON THE ?! rocks ; a hypothesis of thoir origiii which may bo appropriately designated as thermo- chiiotic. Acoording to this hypothesis, as set forth by Scropo, and afterwards by Delabeche ajid by Daubn'-o, th(; first water on the suria<30 of the planet would be condensed under a pressure equal to 250 atmospheres, corresponding to a temperature near that of redness. We are reminded in this of Dana's earlier motamorphic theory, in which he also invoked the action of waters at a red heat. These, however, were supposed by him to be heated in the depths of the ocean by local volcanic eruptions, and the process, so far from being a universal one belonging to a very early time in the history of our planet, was a partial one repeated at different geological periods. According to I)aul>ree the original plutonic rocks are not known, and the oldest crystalline schists arc thennochaotic. Macfarlane, on the contrary, while adopting this hypothesis for the later crystdliue or transition schists, maintains the eudoplutouic origin of the primitive gneisses. § 42. Proceeding now to loview briefly the claims of the above hypotheses, we remark with regard to the first, that multipliiKl observations in many parts of the world have now established the existence of a regular .succession in the crystalline rocks, which show by the greater corrugation of the lower members, by frequt^nt discordances in stratification, and by the presence of fragments of the Iow^m- in the higher strata, that the order of generation was from below upwards. With this, moreover, corre.sponds the fact that the lower rocks are the more massive and more highly crystalline, while the upper ones pre- sent a gradual approximation in physical characters to the uncrystalline sedimentary or secondary strata ; thus justifying the name of transition, applied by Werner to these inter- mediate rocks. All these facts are irreconcilable with the eudoplutouic hypothesis. The universal distribution, and the persistency of charac ters of these various groups of crystalline rocks, indicate moreover that they have been produced by a world-wide action, extending with great regularity throxigli vast periods of time, and are incompatible with anything which we know of the phenomena of vulcanicity. The objections long since made by Naumann to the second or exoplutonio hypothesis are still as valid as ever, and tliere is no evidtnK'e in the lithological characters of these rocks of their volcanic origin. The argument deriv(>d from the similarity between their mineralogical composition and that of erupted rocks, of paleozoic and more recent times, is equally strong in favor of the derivation of these latter from the primitive strata. § 48. The metamorphic hypothesis, whi(Oi would derive the primitive strata from the consolidation and the recrystallization of detrital ])lutonic rocks, whether endoplutonic or volcanic, is, tor many reasons, inadmissible. AVithout at present considering the later crys- talline groups, which are also of vast extent, the ancient granitoid gneisses, (originally called Laurentian and represented in Canada by the Ottawa and Grenville series,) have an unknown A'olume, since their base has never betMi detected. It is, however, certain that th(>y include, wh(>rever studied in Europe or in America, a vast thickness which, as Dana corn-ctly says, m befbre us require moreover a source, neither detrital nor volcanic, for the immense mass of wholly crystalline material, chiefly quartz and feldspars, constituting the vast and as yet unfathomed primitive granitic and gneissic series ; which only at a later time furnished its contingent of decayed and detrital matter to the crystalline transition rocks. But there is still another condition imposed by the problem before us — that of a satis- factory explanation of the highly inclined and often nearly vertical attitude of the crystal- line stratified rocks, which is most remarkable in those of the earliest periods. Tho ordinarily received explanation of this, as due to the contraction of a cooling globe, has seemed so inadequate to account for the great contortion, crushing, and folding of these older rocks, that some geologists, as Naumann tells us, have been led to regard the present as their original attitude, resulting from movements of the solidifying crust ; in which connection he quotes with approval the language of Kittel, that '• so long as a hypothesis is unable thoroughly to explain the almost vertical position of the primitive strata, it cannot be regarded as even approximately near the truth." It will, we think be apparent, in the light of the preceding review of existing hyi>o- theses, that no explanation of the origin of the crystalline rocks which fails to meet all of the conditions just defined can hope for the approval of those who, after a careful survey of the whole field, seek for a new and more satisfactory hypothesis. It remains to be seen *^ The Decay of Rocks Geologically Considered,*1883.— Amor. Jour, Sci., vol, xxvl., pp. 190-213, OBIGIN OF CRYSTALLINE ROCKS. 23 whether, vvllh the help of modern physical and chemical science, and our present knowledge of geological facts, it is possible to devise such a one. After many years of reflection and study, the present writer ventures to propose a new hypothesis, believing that while avoid- ing all the difficulties of those hitherto put forward, it will furnish an intelligible solution of a great number of hitherto unsolved problems in the physiology of the globe. II. — The Development of a New Hypothesis. § 47. The history of the beginning and the growth of the new hypothesis here proposed to explain the origin of crystalline rocks is necessarily to a great extent personal, since it covers the work of many years of the author's life. The lines of investigation which have led to this hypothesis may be described as first, that of the order and succession of the crystalline stratified rocks of the earth's crust ; secondly, their mineralogy and lithology ; thirdly, their history, considered in the light of physics and chemistry, involving an inquiry into all the chemical relations of existing rocks, waters and gases, including the transfor- mations and decay of rocks, and the artificial production of mineral species ; and fourth and lastly, the probable condition of our planet before the creation of the present order. The adequate discussion of all these themes, which would include a complete system of mineral physiology, is impossible within the limits of the present essay, but a brief outli'ie of some of the chief points necessary to the understanding of the hypothesis will here be attcTapted. § 48. As regards the order and succession of the crystalline rocks, the author's studies of them, begun in New England forty years since, and continued in Canada from 1847 onwards, were for many years perplexed with the difficulties of the Huttonian tradition, (then and for many years generally accepted in America) that the mineral character of these rocks was in no obvious way related to their age and geological sequence, but that the strata of paleozoic and even of cenozoic times might take on the forms of the so-called azoic rocks. It was questioned by the partisans of the Huttonian school whether to the south and east of the azoic rocks of the Laurentides and the Adirondacks, in North America, there were any crystalline strata which were not of paleozoic or of mesozoic age, although many of these are undistiuguishable from the rocks of the Laurentides. As I have elsewhere said, the metamorphic and the metasomatic, not less than the exoplutonic hypothesis, of the origin of the crystalline rocks, by failing to recognize the existence and the necessity of an orderly lithological development in time, have powerfully contributed to discourage intelligent geognostical study, and have directed attention rather to details of lithology and of mineralogy, often of secondary importance.*' That a great law presided over the development of the crystalline rocks, was from the first my conviction, but until the confusion which a belief in the miracles of metamorphism, metasomatism, and vulcanism had introduced into geology was dispelled, the discovery of such a law was impossible. § 49. Convinced of the essential truth of the principles laid down by Werner, and embodied in his distinctions of Primitive, Transition and Secondary rocks, I sought, during Amer. Jour. Scienoe, 1880, xix, 298. Sec. III., 1884, 4. 26 DR. THOMAS STERRY HUKT ON THE many years, to defiue and classify the rocks of the first two of these classes, and by ex- tended studies in Europe, as well as in North America, succeeded in establishing an order, a succession, and a nomenclature, which are now beginning to lind recognition on both continents.^' While the succession of the various groups of crystalline rocks was thus being esta- blished, not without the efficient aid and co-operation of other workers in late years, min- eralogical and chemical studies were teaching us much of the true nature of the diffe- rences and resemblances of these groups, as well as of the natural relations and modes oi formation of various silicates and other mineral species which enter into the composition of the crystalline rocks. The investigations of physicists and astronomers had moreover given form and consistence to the ancient theory of the igneous origin of our planet, and the concurrent working in all of the lines of investigation above indicated was thus pre- paring the way for a new hypothesis of the origin of crystalline rocks — a hypothesis of which I shall endeavour to sketch the growth and the evolution. § 50. It was in January, 1858, more than a quarter of a century since, that I ventured to put forth a speculation as to the chemistry of a cooling and still molten globe. Consi- dering only that crust with which geognosy makes us acqiiaiuted, it was maintained that at a very early period the whole of its non-volatile elements were imited in a fused mass of silicates, which included the metiiUic bases of the salts now dissolved in the ocean's waters ; while the dense atmosphere of that time was charged with all the carbon, sulphur, and chlorine, combined with oxygen or with hydrogen, besides which were present watery vapor, nitrogen, and a probable excess of oxygen. The first precipitated and acid waters from this atmosphere falling on the hot earth's silicated crust, would, it was said, soon become neutralized by the protoxyd bases, giving rise to the chlorids and sulphates of the primeval sea ; with the probable separation of the combined silica, at that high tempera- ture, in the form of quartz. The suggestion as to the acid nature of the primitive atmosphere, and its first chemical action, which Avere obvious deductions from the igneous theory, had, as I afterwards learned, been anticipated by Quenstedt.'" § 51. These views were reiterated in May, 1858, when they were coupled with the conception of a solid nucleus to the globe as then taught by Poulett Scrope and by Wil- liam Hopkins. The subsequent subaerial decay of exposed portions of the earth's primi- tive crust in a moist atmosphere, now purged of the acid compounds of chlorine and sulphur, but still holding carbonic acid, was then set forth as resulting in the transforma- tion of feldspathic silicates into clays, and the transference to the sea of the lime, magnesia and alkalies of the decayed rock in the form of carbonates, the latter of which, reacting on calcium-chlorid, would yield carbonate of lime and chlorids of sodium and magnesium. It was then said that by this hypothesis " we obtain a notion of the processes by which, from a primitive fused mass, may be generated the various silicious, argillaceous and calcareous " I liave elsewhere given the history of the progress of inquiry in this direction in Report E of the Second Geological Survey of Pennsylvania (Azoic Rocks) 1878 ; in briof, in an essay on Pr6-Cam1)rian Rocks, etc., in the Amer. Jour. Science, 1880, (xiv. 2fj8) ; and later in a study of the Pre-Camlirian Rocks of the Alps, in the Trans. Roy. Soc, Canada, vol. i, part 3, pp. 182-190. See also in this connection the late address of Dr. Hicks, president of the British Geologists' Association, in ita Proceedings, vol. viii. 1883, On the Succession of the Archasan Rocks, etc. ; and the still more recent paper of Prof. Ronnoy, president of the Geological Society of London, on The Building of the Alps, in Nature for May 18 and 25, 1884 ; also tlie Geological Magazine for June 1884, p. 280. ** Epochen der Natur. i). 20. OEIGIN OF CEYST>LLINE ROCKS. 27 rocks which make up the greater part of the earth's crust." Of this it was declaried, " the earth's solid crust of anhydrous and primitive igneous rock is everywhere deeply concealed beneath its own ruins, which form a gi-eat mass of sedimentary strata, permeated by water," and subjected to heat from below, changing them to crystalline metamorphic rocks, and at length reducing them to a state of igneo-aqueous fusion, through which they yield eruptive rocks. Of this primitive crust it was farther asserted that it " probably approached to dolerite in composition." The principal points in this hypothesis, as presented in 1858, were thus the solid condition of the earth's interior, and the derivation of the whole of the rocks of the known crust, by chemical transformations, from the original superficial and last-congealed layer of the cooling globe, which was considered to have been a basic rock, not unlike dolerite. All of these positions are fundamental to the present hypothesis. § 52. These views were again repeated in a paper read before the Geological Society of London in June, 1859, with some farther developments as to the origin of the various crystalline rocks derived from the primeval crust. This, it was claimed, was necessarily quartzless, and far removed in composition from the supposed granitic substratum, or the primitive gneiss. An attempt was, however, made to show that with the quartz, derived from the supposed first decomposition of the primitive igneous rock by acid waters, and the sediments resulting from subsequent disintegration and subaerial decay, coarser and liner sediments, more or less permeable, would result, which by the natural chemical action of infiltrating waters might, in accordance with known laws, divide themselves into two great classes, " the one characterized by an excess of silica, by the predomin- ance of potash, and by small amounts of lime, magnesia and soda, and represented by the granites and trachytes ; while in the other silica and potash are less abundant, and soda, lime and magnesia prevail, giving rise to pyroxene and triclinic feldspars. The metamorphism and displacement of such sediments may thus enable as to explain the origin of the different varieties of plutonic rocks without calling to our aid the ejections of the central fire." § 53. Such was the scheme put forward by the writer, in 1858 and 1859, to explain the generation from a homogenous undifferentiated crust, without the intervention of plutonic matters from the earth's interior, of the two great types of acidic and basic crystalline rocks ; gneisses, granites and trachytes on the one hand, and doleritic rocks, green- stones and basalts on the other. Regarded as an attempt to adapt the Huttonian hypo- thesis to the growing demands of the science, and to give it what it had hitherto lacked, a starting point in time, and a possible explanation of the two types of acidic and basic rocks, this scheme demands a place in the history of geology, although, in the judgment of its author, it must share the fate of all other forms of the metamorphic hypothesis. In recognizing the adequacy of a primitive undifferentiated layer of igneous rock as the sole source of the materials of the future order it, however, effected a great step towards a more satisfactory hypothesis.^" *' See, for the references to this early statomont, the Amorican .Tournal Science for January, 1858, (vol. xxv, p. 102;) also a Theory of Igneous Rocks and Volcanoes, Canadian .Tournal, Toronto, May, 1858 ; and Some Points in Chemical Geology, in abstract in Philos. Mag. for February, and in full in the Quarterly Geological Journal for November, 1869. The latter two papers are reprinted in the author's Chomieal and Geological Essays, PP' 1-17. 28 DR. THOMAS STERRY HUNT ON THE § 54. The nature and history of this primitive layer was farther discussed by the author iu a lecture ou " The Chemistry of the Primeval Earth," given at the Royal Insti- tution in London, iu June, 1861."' Therein it was said : " It is with the superficial iwrtions of the fused mineral mass of the globe that we have now to do, since there is no good reason for supposing that the deeply-seated portions have intervened in any direct manner in the production of the rocks which form the superficial crust. This, at the time of its first solidification, presented probably an irregular diversified surface, from the result of contraction of the congealing mass, which at last formed a liquid bath of no great depth, surrounding the solid nucleus." It was further insisted that this material would contain all of the bases in the form of silicates, and must have much resembled in composition certain furnace-slags or volcanic products. Of this primary lava-like rock, it was said, that it is now everywhere concealed, and is not to be confounded with the granitic substratum. That granite was a secondary rock, formed through the intervention of water, was then argued from the presence therein, as a constituent element, of quartz, " which, so far as we know can only ])e generated by aqueous agencies, and at compara- tively low temperatures." The metamorphic hypothesis of the origin of granite was then maintained. In 1869, in an essay on "The Probable Seat of Volcanic Action,""'' a further inquiry was made into the probable nature and condition of what had been spoken of in 1858 aa "the ruins of the crust of anhydrous and primitive igneous rock." This, it was now said, " must by contraction in cooling have become porous and permeable, for a consider- able depth, to the waters afterwards precipitated upon its surface. In this way it was pre- pared alike for mechanical disintegration and for the chemical action of the acids .... present in the air and the waters of the time. . . . The earth, air, and water, thus made to react upon each other, constitute the first matters, from which, by mechanical and chemical transformations, the whole mineral world known to us has been produced." It was farther argued, from many geological phenomena, that we have evidence of the exist- ence between the solid nucleus and the stratified rocks of " an interposed layer of partially fluid matter, which is not, however, a still unsolidified portion of the once liquid globe, but consists of the outer part of the congealed primitive mass, disintegrated and modified by chemical and mechanical agencies, impregnated with water, and iu a state of igneo- aqueous fusion." § 55. Although in 1858 I had, as already shown, soiight to give a more rational basis to the metamorphic hypothesis of the origin of crystalline rocks, the traditions of which, as expounded by Lyell, weighef' so heavily on the geologists of the time, other considera- tions soon afterwards led me to seek in another direction for the solution of the problem. The examination of the mineral silicates deposited during the evaporation of many natural waters, that of the Ottawa river among others, and the study which 1 had made of the hydrous magnesiau silicate found in the tertiary strata of the Paris basin, induced me, as early as 1860, to inquire "to what extent rocks composed of calcareous and magnesiau silicates may be directly formed in the moist way ;" and again, in the same year, to declare " Proceedings of the Royal Institution, and also Chomical and Geological Essays, pp. 36-45. " Geological Magazine
lase, by an admixture of which it passes elsewhere into a well characterized granitic vein. The quartz veins, in places, hold cubic crystals of pyrite, together with chalcopyrite and pyrrhotite, the latter in consider- able masses, sometimes accompanied by crystals of greenish epidote, imbedded in the quartz, and occasionally associated with red garnet. In one part, there is found enclosed in the wider portion of a vein, between bands of vitreous quartz, a lenticular mass, three inches thick, of coarsely granular pink calcite, with imbedded grains of dark-green amphi- bole and on one side small crystals of olive-green epidote and red garnet ; the whole mass closely resembling some crystalline limestones from the Laurentian," and evidently endogenous.** I have also described remarkable examples of similar associations of zoisite, garnet, hornblende, pyroxene and calcite in the metalliferous quartz lodes in the Montalban series, at Ducktown, Tennessee.''" § 60. The question of the aqueous origin of concretionary veins was resumed by the author in 1871, in an essay On Granites and Grranite Veinstones, when it was maintained that the relation of granitic veins with metalliferous quartz-lodes, on the one hand, and ''Geology of Canada, 1863 ; pp. 470 and 606. " Azoic Rocks, Report E, Second Geological Survey of PonnRylvanla, p. 247. " Chemical and Geological Essays, p. 217. ORIGIN OP CRYSTALLINE ROCKS. 31 with calcareous veius carrying the ordinary minerals of crystalline limestones, on the other, is such that to all these veins must be assigned a common aqueous origin. It was farther shown that the endogenous granitic masses or veinstones in the Montalban or younger gneissic; series in New England often attain ])readths of sixty feet or more, and that they present great varieties in texture, from coarse aggregates of banded orthoclase and quartz, often with muscovite (from which these various elements are rained for com- mercial purposes), to veins in which the concretionary character is not less marked, including beryl, tourmaline, garnet, cassiterite and other rare minerals ; while others still of these great veins are so iine-grained and homogeneous in character as to have been quarried as granites for architectural uses. These endogenous masses are included alike in the gneisses, the quartzites, the staurolitic mica-schists, and the indigenous crystalline limestones of the Montalban series, and, though generally transverse, are sometimes, for a portion of their course, coincident with the bedding of the enclosing rock."^ It was clear that these endogenous granitic veins of posterior origin were mineralogi- cally very similar to the older gneisses and the erupted granites. From a prolonged study of all these phenomena, the conclusion was then reached that we have in the action which generated these endogenous granitic rocks a continuation of the same process which gave rise to the older or fundamental granitoid gneisses, which were hence of aqueous origin. § 61. This process of reasoning was in fact identical with that by which Werner, in the last century, was led to assign an aqueous origin to the primitive granite and the crystalline schists. In a farther description, in 18'74, of some examples of these banded veinstones from Maine and Nova Hcotia, it was said that their structure is " due to successive deposits from water of crystalline matter on the walls of the vein, and results from a process which, though operating in later times and in subterranean fissures, was probably not very much unlike that which gave rise to the indigenous granitic gneisses."'" The same ideas as to the origin of the ancient crystalline rocks, and their relations to granitic and to zeolitic veins, were farther defined by me, in 18*74, when it was said : "The deposition of immense quantities aliks of orthoclase, albite and oligoclase in veins which are evidently of aqueous origin shows that conditions have existed in which the elements of these mineral species were abundant in solution. The relation between these endogenous deposits and the great beds of orthoclase and triclinic feldspar-rocks is similar to that between veins of calcite and of quartz, and beds of marble and of traver- tine, of quartzite and of hornstone. Eut while the conditions in which these latter mineral species are deposited from solution have been perpetuated to our own time, those of the deposition of feldspars and many other species, whether in veins, or in beds, appear to belong only to remote geological ages, and, at best, are represented in more recent times only by the production of a few zeolitic minerals." "" § 62. A farther and more particularized statement of the author's conclusions as to the origin of the crystalline rocks was embodied in i paper read before the American Associa- tion for the Advancement of Science at tSaratoga, in August, 1879, containing the three following propositions : "' ^ Amer. Jour. Science (3), vol. i., 88 and 182, and vol. iii., 115 ; also, Chem. and Gool. Essays, pp. 183-209. 59 Proc. Boston Society of Natural History, xvi. 237, p- 198. "" Ciiemical and Geological E.«8ays, p. 298. ""The History of some Pre-C'ambrian Rocks, etc. I'roc. A. A. A. S., for 1879, and Amer, Jour. Science (1880) six., p. 270. 82 DB. THOMAS STERRY HUNT ON THE Ist. All gneisses, petrosilexes, hornbleudic and micaceous schists, olivines, serpentines, and in short, all silicated crystalline stratified rocks, are of neptunian origin, and are not primarily due to metamorphosis or to metasomatosis, either of ordinary aqueous sediments or of volcanic materials. 2ud. The chemical and mechanical conditions under which these rocks were deposited and crystallized, whether in shallow waters, or in abyssal depths (where pressure greatly influences chemical affinities), have not been reproduced to any great extent since the beginning of paleozoic time. 3rd. The eruptive rocks, or at least a large portion of them, are softened and displaced portions of these ancient neptunian rocks, of which they retain many of the mineralo- gical and lithological characters. § 63. In a subsequent paper, in 1880, it was said, with reference to the subaerial decay of rocks: "The aluminous silicates in the oldest crystalline rocks occiir in the forms of feldspars, and related species, and are, so to speak, saturated with alkalies or with lime. It is only in more recent formations that we find aluminous silicates either free or with reduced amounts of alkali, as in the argillites and clays, in micaceous minerals like musco- vite, margarodite, damourite and pyrophyllite, and in kyanite, fibrolite and andalusite ; all of which we regard as derived indirectly ""fom the more ancient feldspars." In connection with this important point, which I had already discussed elsewhere, I added the following note, referring at the same time to the propositions of the preceding paragraph : °- " It is a question how far the origin of such crystalline aluminous silicates as muscovite, margarodite, damourite, pyrophyllite, kyanite, fibrolite and andalusite, is to be sought in a process of diagenesis in ordinary aqueous sediments holding the ruins of more or less com- pletely decayed feldspars. Other aluminous rock-forming silicates, such as chlorites and maguesian micas, are, however, connected, through aluminiferous amphiboles, with the non-aluminous maguesian silicates, aud to all of these various magnesian minerals a very different origin must be ascribed." In a farther discussion of this subject, in 1883, it was noted " that decayed feldspars, even when these are reduced to the condition of clays, have not, in most cases, lost the whole of their alkalies." '^' This was shown by the analyses made by Sweet, of the kaolinized granitic gneisses of Wisconsin, from which it appears that " the levigated clays from these decayed rocks still hold, in repeated examples, from two to three hundredths or more of alkalies, the potash predominating." § 64. The question of the source of the matters in aqiieous solution which, according to the hypothesis before us, gave rise to granitic veinstones, naturally comes up at this stage of our inquiry. As we have seen, the granitic substratum of igneous origin, the existence of which is postulated by most modern geologists is, since the time of Scrope, Scheerer and filie de Beaumont, generally conceived to be impregnated with a portion of water, conjectured by Scheerer to equal perhaps five or ten hundreths of its weight ; and through the intervention of this to assume, at temperatures far below the point of liquefac- «' The Chemical and Geological Relations of the Atmosphere, Amer. Jour. Science, xix, 354. See farther, for the stratigraphical relations of the various aluminous silicates, (which were first set forth by the author in 1863), Chem. and Geol. Essays, pp. 27 and 28 ; also Report E, Second Geological Survey of Pennsylvania, (1878) p. 210. •"The Decay of Rocks Geologically Considered, Amer. Journal Science, (1883) xxvi, 194. ORIGIN OF CRYSTALLINE ROCKS. 33 tion of tho anhydrowH rock, a condition which has been desij^nated one of aqueo-iirneonR fusion. This interposed water, under the inlluence of <^reat heat and pressnre, \v(! may suppose, with Scheeror, to constitute a sort of granitic juice, which, oxiiding from the mass, might fill fissures or other cavities, alike in the granite and in the adjacent rocks, with the characteristic minerals of granitic; veins. This st>oms to have been essentially the view of filie de Beaxaraont, who described the elements of the p(>graatites, the tourmaline- granites, and the veins, often abounding in quartz, which carry cassiterite and columbite, as emanations from the adjacent granitic masses, or as a granitic aura. Daubreo and Scheerer, in previously describing the similar granitic; veins found in Scandinavia, con- ceived them to have been filled in like manner, not from an nnstratified granitic sub- stratum, but from the crystalline schists which enclose them.'' § 65. In both of the above hypotheses, we note that tho source of tho orthoclase and the quartz of the veins is sought in the solutions derived from the granitic substratum or its closely related crystalline schists. If now we go farther back, and ask for the origin of this granitic substratum, with its constituent minerals, wo have shown, in opposition to the view that it is the outer layer of a cooling globe, good reasons for maintaining, in the first place that such a layer must have had a very dili'erent composition from that of gra- nite, and in the second place that granite itself is a rock of secondary origin, in the forma- tions of which water has in all cases intervened. We have, moreover, already sought to show that the attempt to derive this granitic rock, by any process of metamorphosis or metasomatosis, from sediments formed from the primitive quartzless rock, was untenable, and that the vast granitic substratum, so homogeneous and so widely spread, could not thus have originated. Already, in 1874, it had been declared that the process which generated the orthoclase and the quartz of tho granitic rocks was one represented in more recent times by the production of zeolites. § 66. The generation from basic rocks, by aqueous action, alike of orthoclase, of quartz, and of zeolites, is well known. These are often as.sociated in such rocks, under conditions which show them to be secretions from tho surrounding mass. The substance named palagonite is an amorphous, apparently colloidal, hydrous silicate, the composition of which, deducting the water (about seventeen per cent, on an average), is, according to Bunson, identical with that of his normal pyroxenic or basaltic magma (§ 24), except that the iron in palagonite is in the state of peroxyd. This sul)stance is changed by no great eleva- tion of temperature into the zeolite, chabazite, a crystalline silicate of alumina and alkalies, rich in silica, but destitute of iron-oxyd and magnesia, and a more basic residuum, in which the latter two bases are retained. Basaltic rock is, according to Bunsen's observations in Iceland, changed through hydration into palagonite, " under the influence of a ueptunian cause," and this, by the heat of contiguous eruptive masses, is subsequently transformed into a zeolitic amygdaloid. These operations, as he has shown, may be repeated in our laboratories. Fragments of amorphous native palagonite, when rapidly heated in tho flame of a lamp, develope in their mass cavities filled with a white matter, recognized by the aid of a lens as crystalline chabazite ; while the transformation of basaltic rock into palago- " For a general account of the views described in this paragraph, and for references to the somewhat extended literature of the subjeot, see Hunt, Chemical and Geological Essays, pp. 188-191 ; also Ibid., p. 0. ^ Sec m., 1884. 5. 34 DR. THOMAS STERRY HUNT ON THE nite itself may also bo artificially effected." Palagonite, is not, apparently, a distinct mineral species, but a colloidal hydrated mixture, interesting as marking a stage in the transforma- tion of the vitreous form of certain basic silicated compounds. The crystalline forms of these by their decomposition may, however, yield zeolites without passing through this intermediate stage. § 6*7. That in these curious but neglected observations of Bunsen's, wo have repro- duced in miniature not only the process which takes place on the large scale in masses of basic exoplutonic rock, but the process which must have gone on in the early ages, when the universal basic rock, which we have supposed to form the surface of the cooling globe, was heated from below, and penetrated by atmospheric waters — was a deduction which, although it seemed legitimate, was too vast and too far-reaching to be lightly accepted. It was therefore not until after many years of careful consideration, and the examination and rejection of all other conceivable hypotheses, that the conviction was aciquired that in these reactions, which give rise to zeolitic minerals, we have the true solution of the pro- blem of the genesis of crystalline rocks. This was formally enunciated in 1884, when, after considering the condition of a cooling earth, in accordance with the hypothesis defined in § 50, it was said: "The globe, consolidating at the centre, left a superficial layer of matter, which has yielded all the elements of the earth's crust. This last-cooled layer, mechanically disintegrated, saturated with water, and heated by the central mass, furnished in aqueous solution the silicates which were the origin of the ancient gneisses and similar rocks." " "Tl)o following is the composition assigned by Biinsen to the typical trachytic and basaltic magmas, and to palagonito, as deduced from his studies of tiiose tckU in Iceland ; A, being the normal trachytic tyiw, the moan of seven analyses of trachyte and obsidian ; B, the normal basaltic typo, from six analyses of basalt and lava; and C, the average of several palagonites of that region, deductiiiL' tiio water:— A. B. G Silica 70.67 48.47 49.15 Alumina 11.15 . 14.78 ■» Ferrous oxyd 3.07 " 15.38 |30.82 Limo 1-45 11.87 9.78 Magnesia 0.28 6.89 7.97 Totash 3.20 0.65 0.99 Soda 4.18 1.96 1.34 100.00 100 00 100.00 The ferrous oxyd in the six examples from which B was deduced varietl from 11.69 to 19.43 ; while for the palagonite, the iron (which is not separated from the alumina in the above average, and is present 'as ferric oxyd ) ranged from 11.85 to 21 .30. The wat«r therein varied from 16.0 to 24.0 per cent. The oxygen ratio for pall- gonite, takmg the maximum of alumina, 18.97, and the ferric oxyd, 11.85, together, would be about 1 • 2 • 4 • and excluding the latter from the calculation, very nearly 1:1|:4. Palagonite, according to Bunsen is thus a hydrated baaalt which hus exchanged a portion of its lime for magnesia, with poroxydation of the contained iron. It "IS the amorphous portion of basalt that gelatinizes with acids, which is the part forming zeolites" (correspondmg to the vitreous matter of the tachylite-basalts), and the hydration of this yields palagonite Bunsen, by fusing a basalt with potassic hydrate, and treating the mass with water, got a material which differed from the basalt only in having lost a little silica and acquired 30.0 of water, and which had all the charac- ters of palagonite. (Bunsen, Recherches sur la formation des roches volcaniques en Islanda Ann. do Chim et de Phys. (1853) (3) xxxviii., 215-289. "From a report of a lecture by the author before the Lowell Institute, Boston, Mass., Feb. 29 1884 in the Boston Daily Advertiser of March 1. , ' ' ORIGIN OP CRYSTALLINK ROOKS. 38 § 68. The trauBformation of the primary basic layer, judging from the phenomena seen in basic exoplutonic roi'ks, would give rise not only to quartz, feldspars and zeolites, but to aluminous silicates like opidote, chlorastrolite and prehnite, and to non-aluminous silicates like pectolite, okenite and apophyllite. These siliiates are all non-muguf.sian, but their reactions, while in a soluble condition, with dissolved magncsian salts would give rise to various natural magnesiau silicates, both aluminous and non-aluminous. § 69. The cooling of the surface of the earth by ratliation, and the heating from below, would establish in th(! disintegrated, porous and unstratilied mass of the primary layer a system of aqueous circulation, by which the waters penetrating this permijablo layer would be returned again to the surface as thermal springs, charged with various matters there to be deposited. The result of this process of upward lixiviation of the mass would be the gradual separation of the primary undifferentiated layer into an upper stratum, consisting chiefly of acidic silicates, such as feldspars with qtiartz, and a lower, more basic and insoluble residual stratum, charged with iroxi and magnesia ; the two re- presenting respectively the overlying granitic and the underlying basaltic layer, the presence of which beneath the earth's surface has generally been inferred from exoplutonic phenomena. The intervention of the argillaceous produ(;ts of subacrial decay was considered, and the reactions between them and mineral solutions from below, it was conjectured, might give rise to certain micaceous minerals. § *rO. That the great shrinking of the primary layer, consequent upoi. ' removal from it, by solution, of the vast amount of matter which built up the overlyiiijr l raniticand gneissic series, would result in a collapse and a general corrugation of i. )verlying deposit, and that this would probably bo attended by outflows, through fissures, of the underlying basic magma, constituting the first eruptive or exoplutonic rocks, were among the most obvious deductions from this hypothesis. These various points were concisely set forth in notes read in April and May of this year, with the suggestion that this newly proposed explanation of the origin of crystalline rocks, through the action of springs bringing up mineral matters from below, might be called the CRENITIC hypothesis, from the Greek xpt'fvif, a fountain or spring."^ § *71. The steps in the chronological history of the new hypothesis, which we have sketched in the preceding pages, may be briefly resumed as follows : — I. — 1858. An attempt to deduce from the doctrine of a solid incandescent nucleus, and a single primary igneous rock, supposed to be quartzless and basic, through mechanical and chemical agencies, two distinct and unlike classes of sedimentary deposits, which, when subsequently transformed by subterranean heat, should give the two types of acidic and basic crystalline rocks. This was an attempt to adopt the Huttonian metamorphic hypothesis to the conception of a cooling globe, and to give it, what it wanted, a point of departure. II. — 1860. An attempt to explain the production, by aqueous action at the earth's sur- face, of various protoxyd-silicates. III. — 1863. An attempt to extend this last conception to double aluminous silicates, by a consideration of the formation of zeolites at the earth's surface in rocks of secondary age, " On the Origin of the Crystalline Rocks, National Academy of Sciences, AVashington, April 15, 1884, in American Naturalist for June; also Royal Society of CanaJa, Ottawa, May 20, in Amer. Jour. Science, July, 1884, and Nature, July 3, p. 227. 36 DR. THOMAS ST EERY HUNT ON THE and also in more recent times, through the action of thermal waters ; it being shown, from the association of zeolites with feldspar and quartz in nature, that all these are sometimes formed contemporaneously from aqueous solutions, and also thai many feldspathic veins and masses have probably had a similar aqueous origin. IV.— 1871. The subject of granite veins, farther discussed, and the mineralogical similarity between these endogenous masses and the indigenous gneissic and granitic rocks insisted upon. v.— 1874. The argument reiterated, that the conditions under which the primitive granitic and gnessic rocks were produced were essentially similar to those of the granitic veins of the later crystalline schists, and that these conditions are reproduced to a smaller extent, in later times, in the formation of zeolitic minerals : finally, that the gneisses and bedded granites are to granitic veins what beds of chemically-deposited limestone and travertine are to calcareous veins. VI.— 1880. The definite assertion of the aqueous origin of stratified crystalline rocks, coupled with the rejection of the doctrines of metamorphism and metasomatism in explain- ing their origin, and the assertion of their pre-paleozoic age. At the same time, the proba- ble intervention of clays, from the subaerial decay of feldpars, as a source of certain crystal- line aluminous silicates is suggested. VII.— 1884. The definite assertion is made that the ancient crystalline rocks were generated either directly from materials brought to the surface by subterranean springs from the primary igneous rock, or, as was the case in later times, by the reactions of these materials with the products of subaerial decay. These latter iiutluded clays from feldspars, and dissolved magnesian salts formed by the action upon sea-water of maguesian car- bonate set free in the atmospb'^ric decomposition of basic rock erupted from the primary stratum. Thus, while what may be called the Trimitive crystalline rocks were wholly crenitic in their origin, the soluble and in.soluble results of the subaerial decay, alike of basic exoplutouic matter, and of the older crenitic rocks, contributed to the formation of the later, or Transition crystalline schists. III. Illustp iTioNs OF THE Crenitic HvrOTHESIS. § 72. The crenitic hypothesis, which has been proposed in the second part of this essay to account for the origin of the granites and crystalline schists, conceives thorn to have been derived, directly or indirectly, by solution from a primary stratum of basic rock, the last congealed and superlicial portion of the cooling globe, through the intervention of circulating subterranean waters, by which the mineral elements were brought to the sur- face. This view not only compares the generation of the constituent minerals of the primitive rocks with that of the minerals formed in the basic eruptive rocks of later times, but supposes these rocks to be extruded portions of the primary stratum which, though more or less modified by secular changes, still exhibited after eruption, though on a limited scale, the phenomena presented by that stratum in remoter ages. The study of these rocks, and of their accompanying secondary minerals, which may be properly described as the secretions of these rocks, will therefore be found very important as illustrations of the crenitic hypothesis. "HW* ORIGIN OF CRYSTALLINE ROCKS. 37 § 13. "Without here entering into the details of their geognosy or their lithology, it is sufficient to recall the fact that such basic eruptive rocks abounding in zoolitic minerals are found, with many characters in common, from the time of the Cambrian or pre-Cambrian Keweenian series of Lake Superior to that of the trias of eastern North America, the tertiary of Colorado and the British islands, and the recent lavas of Iceland. The secreted minerals of these rocks often occur in closed cavities in tufaccous beds, constituting amygdaloids, and, at other times, in veins or fissures of considerable size. They are not, however, con- fined to the tufaceous or recomposed detrital exoplutonic rocks, (which are sometimes themselves hydrated and transformed into palagonite, as described by Bunsen in Iceland,) but occur in veins and cavities in massive rocks, as is well scc.i in the diabase of Bergen Hill, New Jersey, and the massive basalt of Table Mountain, Colorado, both remarkable for their zeolitic minerals. ^ 74. The accumulations of secreted minerals in these conditions are often consi- derable in amount. Among other examples, it may be noticed that the zoolitic masses in the amygdaloids of the Faroe Islands are sometimes three or four feet in diameter, and constitute a large portion of the rock. Veins of laumontite in Nova Scotia attain breadths of a foot or more, while some veins on Lake Superior, which are made up to a great extent of zeolitic and related species, are two and three feet or more in breadth, and often of considerable extent. The history of the chemical composition of the zeoiitc-l)eaving rocks of Lake Superior, and of the changes which have taken place in their degradation from the original eruptive mass, have been studied in d(>tail by Purapelly, with the help of the previous analyses of Macfarlane, but cannot here be discussed.'''* § *75. We must here notice the modes of occurrence of the zeolites of Table Mountain, Colorado, as described in 1882 by Messrs. Cross and Hildebrand."' The tipper forty feet of a great flow of basalt, one hundred feet or more in thickness, shows many cavities, large and small, described as more or less flattened and drawn out. Some of these cavities are empty, while others are more or less completely filled by various zeolites, which are also found in fissures in the mass and, in the case of analcite, in a conglomerate made up of pebbles of basic eruptive rocks, underlying the bed of basalt. The zeolitic deposit often appears as " a reddish-yellow sandstone-like material, which occurs in many of the cavities. In the larger ones it takes the form of a floor, the upper surface being horizontal, and the deposit may be several inches in thickness. Small cavities have been completely filled with it, and it is clear that the deposition has taken place from the bottom of each cavity, upward. In parts of South Table Mountain, especially, the same material has filled fissures. Usually the lower part of such masses is composed of a reddish-yellow mineral in irregular grains, which form a compact aggregate, in which lie isolated spherules of a similarly-colored radiated mineral. These spherules are seldom more than two millimetres in diameter, and are very perfect spheres. They increase in number upwards, and finally form the greater part of the deposit. In one cavity, six or eight feet in horizontal diameter and about two feet high, the deposit is quite diilerent. Here the main mass is loosely granular, and is formed chiefly by a bright greenish-yellow mineral, while a stratified appearance is produced by •" T. Macfai-lano, Geological Survey of Canada, 1800, pp. 140-104 ; Pumpolly, Geology of Michigan, 1872. part 2 ; also the same, on The Motaaoinatic Development of tlio Copper-bearing Kocks of Lake Superior, Proc, Amer. Acad., Boston, (1870) vol. xiii, pp. 253-309. "• Cross and Hildebrand, American Journal of Science, xxiii., 452, anil xxiv., 129. 88 DR. THOMAS STBRRY HUNT ON THE layers of a white or colorless mineral. Some of the white layers are chiefly made up of easily recognized stilbite, and the same mineral, in distinct tablets, forms the upper layer of the whole deposit. There are also irregular seams of white running through the yellow mineral." The greenish-yellow crystalline mineral was found to consist of laumontite, and the other layers were mixtures of stilbite and laumontite, with some of which were found sphe- rules of thomsonite. This, in other ca^nties, formed layers by itself, without admixture of the other zeolites mentioned. The presence of these zeolites in cavities side by side with other cavities which were entirely empty, is, according to the writers whom we have quoted, apparently due to the fact that the former communicated with fissures which were channels for the percolating waters that deposited the zeolites. Such fissures, filled up with similar zeolites, were in many cases found leading to these cavities. § *16. The eruptive rocks which break through the Trenton (Ordovician) limestone at and near Montreal, in Canada, are of various ages and unlike composition. Some of these are highly basic, and have been described as dolerites and diorites, while some have been found to contain analcite, and others again much nephelite, and have been referred to teschenite and nepheline-syenite. In some fine-grained amygdaloidal varieties of these basic rocks, which have been designated dolerites, I long since described the occurrence of heulandite, chabazito, analcite and natrolite, with quartz and epidote.^" These zeolites are not abundant, but in certain of the basic doleritic rocks on Mount Royal I have found remarkable A^eins of orthoclase with quartz and other minerals, which merit a notice in this connection. Included in vertical dykes of these rocks, themselves cutting the horizontal limestones which appear at the base of the mountain, are frequent granitic veins, some- times twelve inches or more in breadth, parallel with the walls of the inclosing dyke, often distinctly banded, and exhibiting a bilateral symmetry which, together with their drusy structure, shews them to be endogenous. The most characteristic of these veins are made up of white, coarsely-crystalline orthoclase with a little quartz which, in druses, presents pyramidal forms. In some of the veins. Dr. Harrington has since detected, besides orthoclase and quartz, nephelite, sodalite, caucrinite, hornblende, acmite, biotite and magnetite. All of these minerals are seemingly secretions from the enclosing basic exotic rock. § YY. The mineral secretions of the basic eruptive rocks may be conveniently grouped under seven heads, as follows : — 1. The aluminous silicates, including the zeolites properly su-called, to which we append the related hydrous species, prehnite and chlorastrolite, and the associated anhydrous species, orthoclase and epidote, which are common in the amygdaloidal rocks of Lake Superior. To those we must add albite, axinite, tourmaline and sphene, observed by Emerson, in 1882, in a diabase dyke in the trias at Deerfield, Massachusetts,'' and also the various anhydrous aluminous silicates found with orthoclase in the veins on Mount Royal, just described. 2. The group of hydrous protoxyd-silicates, the bases of which are lime and alkalies, and '"Hunt, in Geology of Canada, 1863, pp. 441, 655 and 668 ; also Harrington, Report Geol. Survey of Canada, 1877-78, p. 43, G. " Emoraon, Amor. Jour. Scionco, xxiv. pp. 195, 270 and 329. AVo reserve for another occasion the discussion of the paragonesis of the muiorals of tliis locality, so carefully studied by Einenson. ORIGIN or CEYSTALLINE ROCKS. 39 of which pectolite may be taken as the type. These species are sometimes wrongly spoken of as belonging to the class of zeolites. As an appendage to this group, we note the hydrous borosilicate of lime, datolite, frequently found in these rocks. Mention should here also be made of the anhydrous protoxyd-silicates, hornblende and acmite, in the feldspathic veins of Mount Royal. "We have already called attention to the occurrence of hornblende and pyroxene in granitic veins under other conditions (§ 5*7) . 3. Quartz in its various crystalline and crypto-crystalline forms, as rock-crystal, amethyst, chalcedony, agate and jaspery varieties, is found both alone and associated with the minerals of the preceding groups. Hyalite of very recent origin has also been observed by Emerson at Deer field. 4. The oxyds, magnetite andhematite, are frequent in the zeolite-bearing rocks of Nova Scotia, where both of these species form veins in amygdaloid, and where magnetite moreover occurs in drusy cavities with quartz, laumontite and calcite. Hematite, in the form of plates of specular ore, is also found there in veins with laumontite, and manganese oxyd is sometimes associated with these iron-oxyds. Small crystals of hematite on prehnite, with a little manganese oxyd, have been observed by Emerson at the Deerfield locality, as also cuprite on datolite, and malachite on prehnite. In similar associations he, moreover, found small portions of various suljihids, such as chalcopyrite, pyrite, sphalerite and galenite. 5. The presence of native copper, and occasionally of native silver, associated with the various silicates already named, should also be noticed. The former metal is common to the zeolitic rocks of Lake Superior and Nova Scotia. 6. Mention should here be made of the saponite often found in amygdaloidal rocks, which, in its purer form, is a hydrous silicate of magnesia, with but little alumina or iron- oxyd. Matters, apparently of this class fill, or more frequently line, amygdaloidal cavities which are filled with other species. This magnesian hydrous silicate is perhaps distinct in origin from the delessite or iron-chloritc which is a frequent constituent of many basic rocks, such as the melaphyres of Lake Superior, and is probably not a secretion but a residual product of the transformation of the rock. T. Calcite in various forms is a common species in the rocks in question, and fluorite and barytine may also be mentioned as accidental minerals therein. It is principally with the first two classes of minerals, the zeolitic group, with its appendages, and the pectolitic group that we have to do. These two, as is well known, though chiefly found in the eruptive rocks already noticed, are not confined to them. Some species of zeolites occur occasionally in veins in gneiss and other crystalline rocks, and even in limestones and other sedimentary deposits. These occurrences are the more readily understood when we consider that the same minerals have been recently formed by the action of thermal waters in various localities, and are even generated in sub-marine ooze. Many of the species of these two groups have also been formed artificially in the chemist's laboratory. § 78. It is our present purpose to consider, first, the zeolitic, and secondly, the pectolitic group, both as regards their chemical composition and their relations to various anhydrous silicates. We shall then proceed to notice the action of water at high temperatures on glass and similar bodies, in giving rise to various crystalline species, including quartz. In this connection will also be discussed some facts relating to the chemistry >f the alkaline 40 DE. THOMAS STEBBY HUNT ON THE silicates. "We shall next notice the action of thermal waters in historic times, and the occurrence of zeolites in the clays of the deep sea, and then pass to the experiments on the artificial reproduction of zeolitic species in the laboratory of the chemist, and discuss the relations of hydrous and anhydrous species. From this, we shall proceed to a consideration of the reactions of the hydrous species of the two groups with raagnesian salts. The origin TABLE OF ZEOLITES AND RELATED SPECIES. HvDKOus. R : r : Si : H Anhydrous. Thomsonite 1:3:4:2 Ca.Na. Anorthito, Paranthito, Sodalite, Gismondite 1:3 : 4J : 4J Ca. Nephelite. Esmarkito 1:3:5:1 1:3:5:2 Mg. Mg. V Barsowite, Bytownite, lolite. Natrolito Scoloc'ito Mosolito Lovynito 1:3:6:2 1:3:6:3 1:3:6:3 1:3:6:4 Na. Ca. Ca, Na. Ca, Na. \ Labradorito. ) Analcito Eudnophite Edingtonite. Laumontite Herschelito Philliijsite Chabazito Onhlflnito 1:3:8:2 1:3:8:2 1:3:8:2 1:3:8:4 1:3:8:5 1:3:8:5 1:3:8:6 1:3:8:6 Na, Ca, K. Na. Ba. Ca. Na,K. Ca, K. Ca, Na, K. Ca, Na. ) Hyalophano, Andosito, Loucito. Faujasite Hypostilbilo Puflerite 1:3:9:5 1:3:9:6 1:3:9:6 Ca, Na. Ca, Na. Ca. > Oligoclase. ; Harmotome 1 : 3 : 10 : 5 Ba. . ? Heulandite Epistilbito Brewsterito Stilbite 1:3 : 12 : 5 1:3 : 12 : 5 1:3 : 12 : 5 1 : 3 : 12 : 6 Ca. Ca. Ba, Sr. Ca. \ Orthoclaso, Albito. Prehnite 2:3:6:1 Ca. ? Chlorastrolite 1:2:3:1 Ca, Na. Epidote, Zoisite, Meionite. of these salts through subaorial decay of exoplutonic magnesian silicates, and their relation to the primeval sea, will then claim our notice ; after which will be considered the probable relations of the clays from the subaerial decay of feldspathic rocks to other classes of rock- ■■m mmm ORIGIN OF CRYSTALLINE ROCKS. 41 making silicates. The conditions of crystallization of mineral matter will next be consi- dered in relation to I he formation of rocks, after which the conclusions of our present study will be briefly summed up in the fourth and last part of this essay. § "79. In the accompanying table of zeolites and related species, are placed, in the first column, the names of hydrous species ; in the second column are given the oxygen-ratios between the protoxyd-bases, the alumina, the silica, and the water, represented respectively bv R, r, Si, and H , while in the third column, appear the symbols of the predominant pro- toxyd-bases in the respective species. In the fourth column are gi^^en the names of corres- ponding anhydrous species, the protoxyd-bases of which are too well known to require designation. In this and the succeeding tables I have generally followed the terminology and adopted the formulas given in the fifth edition of Dana's " System of Mineralogy." In the line with the most basic zeolite known, thomsonite, are placed not only the feldspar, anorthite, but a scapolitic species, paranthito, and sodalite. The minerals of the sodalite group, including hauyine and nosite, correspond, as is well known, to a silicate of the anorthile type united with a chlorid or a sulphate. With nephelite is coupled the hydrous species gismondite, a true zeolite. The recent analyses, by Cross and Hildebraud of the zeolites of Table Mountain, Colorado, give for the zeolites having the characters of thomsonite a proportion of silica greater than corresponds to the formula of that mineral given by Eammelsberg, which we have placed in the table. Some of their analyses, while yielding almost exactly the other ratios of the formula, give for silica, instead of 400, the numbers, 465, 4'76 and even 517 ; showing a composition more silicious than that of gis- mondite, and approaching that of a zeolite corresponding to fahlunite, bar.sovvite and bytownite. These chemists, while believing the specimens analyzed by them to represent a i^ure and unmixed mineral, leave undecided the question of its real composition. § 80. The feldspars, barsowite and bytownite, according to several concordant analyses, are as distinct from anorthite as they are from labradorite, and apparently as much entitled to form a distinct sjiecies as the latter feldspar, or as andesiteoroligoclase. The composition of a lime-barsowite, with the ratios, 1:3:5, would be silica 48.54, alumina 33.33, and lime 18.13 = 100.00. With these feldspathic muierals has been placed iolite, which is a magnesia-iron silicate, giving the above ratios and, as I long since pointed out, is from its atomic volume entitled to be regarded as a feldspathide. With these various anhydrous species would appear to correspond A'ery nearly the so-called thomsonite of Cross and Hildebrand. With this anhydrous group we have placed two hydrous maguesian species, the one, esmarkite, also called praseolite and aspiisiolitc, ar-1 the other fahlunite, which includes what have been called auralite and bonsdorflite. Thet>e species are often associated in nature with iolite, from which they differ only in the presence of water, a)id they have been by most mineralogists regarded as formed by subsequent hydration from this mineral. This view, however, was contested by Scheerer, who regarded the association of the hydrous and anhydrous minerals, as due to a simultaneous crystallization of two isomorphous species." The relations of the silicates of the natrolite section to labradorite are obvious from the table. The same may be said of the relations of the numerous silicates of the anal- cite section to andesite, hyalophane and leucite, and of the faujasite section to oligoclase. " Amor. Jour. Scionco (1848), v. 385, from Pogg. Annalon, Ixviii, 319. Soo. III., 1884. 0. 42 DR THOMAS STERRY HUNT ON THE It is to be noted that the well-defined zeolite, harmotome has as yet no corresponding anhydrous silicate. Of the heulandite section, and the corresponding feldspars, orthoclase and albite, it is to be remarked that orthoclase and albite are the only feldspars hitherto found associated with zeolites, and the only i'eldspars as yetartifKially produced in the wet way. The observations of Whitney already noticed (^ HI) have since been fully confirmed by Tumpelly, who finds orthoclase very common with the zeolitic minerals on Lake Supe- rior, where its deposition is shown to be posterior to laumontite, prehnite, analcite, apo- phyllite, quartz, calcite, copper and datolite ; the only species superimposed upon it being calcite, chlorite and epidote, Avhich latter also occasionally occurs between laumontite and prehnite, in order of superposition." § 81. We have placed at the end of the table the two hydrous silicates prehnite and chlorastrolite which, from their associations, arc evidently, secretions of basic rocks, like the zeolites, though neither of them present the ratios for protoxyds and alumina which characterize these silicates. Prehnite has no known corresponding anhydrous silicate, while chlorastrolite, though a loss common species, is interesting, inasmuch as it affords the oxygen-ratios of the anhydrous species, epidote and zoisite or saussurite ; a fact of some significance in connection with the abundance of epidote in the amygdaloids of Lake Superior. It has also the oxygon-ratios of meionite of the scapolite gi'oup, an anhydrous silicate, which however belongs to a mxich loss condensed type than zoisite, as is indicated by its inferior density and hardness, and its ready decomposition by acids. I have else- whore discussed the relations of those two silicates, and haA^e shewn that the density, hardness, and chemical indifference of epidote and saussurite assign them a place with garnet and idocrase, in the grenatide group ; while meionite, though lacking the proper feld- spar-ratio between protoxyds and alumina, belongs to the foldspathidos."' § 82. It is to be noted that the protoxyd-bases of the zeolites and their related folds- pathidos are either alkalies or lime, baryta or stroutia, if we except the partially magnesian zeolites, picranalcite and picrothomsonite, and iolite and its related hydrous species, which, besides magnesia, include ferrous oxyd. The latter base enters also to some extent into epidote and prehnite. It should also be remarked that small portions of ferric oxyd are frequently found in the analyses of zeolites, amounting, in the red varieties of laumon- tite to three or four, and in some natrolites to one and two hundredths. Some part of this, however, is disseminated in the form of hematite, giving color to the zeolites, and recalling the association alike of hematite and magnetite with zeolites, as already noticed, and a similar occurrence of these oxyds crystallized in many granitic veins. § 83. We next come to the hydrous silicates of lime and alkalies, which we have called, for convenience, the pectolitic group, and which are correlated in the accompanying table with other protoxyd-silicates having similar oxygen-ratios, chiefly magnesian, and partly hydrated and partly anhydrous. We have indicated in the second column, for the known silicates of the pectolitic group, the oxygen-ratios of E, Si, and H, as in the former table, and have left a blank under H, where, as in the first three terms, for example, no pectolitic or non-magnesian species is known. The first place in the table is given to chondrodite, the most basic natural protoxyd- silicate known, and remarkable for the replacement of a small and variable proportion of " See Pampolly, Geology of Michigan, already cited § 74; also Amor. Journal Science, (1871) iii, 254. " Chemical and Geological Essays, pp. 445-447. ORIGIN OF CEYSTALLINE ROCKS. 48 oxygen by fluorine. lu the second line, we find, besides montioellito and chrysolite (including the pure magnesiau variety forsterite or boltonite), the hydrous species, villar- site. With these, moreover, belong the mauganesian species, tephroite ; the zincic, wille- mite ; and the glucinic, phenacite. In the third line, the hydrous silicate, serpentine, with the ratios, 4:3:2, stands alone. Serpentine, unlike villarsite, has no corresponding anhy- drous magnesiau species, and it is worthy of note that, as Daubree has shown, when dehy- drated and fused, it breaks up into chrysolite and enstatite, between whiih, excluding water, it holds an intermediate position."' Dewcylite, in like manner, another hydrous magnesiau silicate with the ratios, 2:8:1, has no corresponding anhydrous species, but is represented by the hydrous lime-silicate, gyrolite, the most basic of the peclolitic group as yet known. TABLE OF PROTOXYD SILICATES. Pi'XrroMTif. K : Si : II AnUYDUOUS and MA(l.Nl«rAN. 4 : 3 ChondrcKlito. 1 : 1 Montioellito, Clirysolito, Toi)liroito, Villarsito. 3 : 4 2:3:1 Serpentine. Gyrolito Uewoylito, Nickol-jrymnito. Xonaltito . . 1:2:1 1:2:2 / WoUnstonito, Enstatito, Hornblende, Pyroxene, \ Rhodonite, Picrosmine, Aplirodita, Corolito. Ploinbiorito Poctolite 5 : 12 : ] Some Ilornblondo? ? 2 : 5 Some Talc. 7 1 : 3 Sopiolite, some Talc. (Unnamed) 1 :4: i 1:4:2 1:4:2 j Apophyllito § 84. We come next to the great section of bisilicates, represented among anhydrous species by wollastonite, enstatite, pyroxene, many hornblendes, and the mauganesian species rhodonite, with many related species and sub-species. With these are the hydrous magne- siau bisilicates, picrosmine, aphrodite, and cerolite, in which the oxygen-ratios, K : Si : H, are respectively 1 : 2 : J ; 1 : 2 : | ; and 1 : 2 : 1|. These various bisilicates are represented among the pectolitic group by plombierite and xonaltite ; the former a lime-silicate found by Daubree in the process of formation at the hot spring of Tlombiercs in Franco, and having the oxygen-ratio, 1:2:2. Of the less hydrated xonaltite, it is worthy of remark that, as observed by Rammelsbcrg, it occurs in concentric layers with the anhydrous species, rhodonite (bustamite), and the hydrous quadrisilicate, apophyllite. While many hornblendes have the ratio of a bisilicate, others are believed to have a "> Compte Bendu de I'Acad. des Sciences, Ixii., le 19 mara, 18G6. 44 DE. THOMAS STERRY HUNT ON THE ratio (excluding a little water) of 4:9, not far from that of pectolite, with which we have placed them. DiJrereut analyses have assigned to talc the ratios for the fixed basis of 2 : 5 and 1 : 3, (the water being variable), — the latter corresponding to sepiolite, 1 : 3 : 1. For neither of these do we know any corresponding pectolitic silicate. § 85. We come, in the last place, to the quadrisilicutes, for which we have no repre- sentatives in the table among anhydrous or among hydroixs magnesian species. They are, however, represented in the pectolitic group by no less than three species, okenite, apophyllite, and an unnamed species got artificially by Daubree. It is fibrous like okenite, is decomposed by acids, and is a hydrous silicate of lime, with six per cent, of soda, giving the ratios, 1:4: i. Pectolite, it will be recollected, contains in like manner about nine per cent, of soda, while apophyllite contains five per cent, of potash and a little fluorine. § 86. The process by which this unnamed pectolitic silicate was obtained by Daubr6e is very instructive, as showing, in many ways, the action of heated water on an undifferen- tiated sili Jahresbericht der Chemie, 1862, p. 128. ^ Way, On the Power of Soils to absorb Manures, Trans. Royal Soc. Agriculture, 1852, xiii, 123-143. '' Cited in Gmelin's Handbook, iii, 439. In, OKIGIN OF CliYSTALLINK ROCKS. 49 te. p8, 111. gelatlnoiiR precipitato, which in soalcd tubes, at toraporaturos of from l.-iO" to 200' C, was gradually .hanged into h.-xagonal plalrs of a pota.sh-«oda z.-oliti, with the oxygen- ratios, 1:8:0:2; having the physie, which is of special signifiance in the aluminous double silicates, is that of their greater or less condensation, or in other words, the relation of their density to their empirical equivalent weight, as already pointed out in the case of the " Dana's System of Mineralogy, 5tli al., p. 479. 54 DE. THOMAS STT'^llY HUNT ON THE scapolite and epidote groups (§ 81).»'" The greater stability of those which belong to the more coudensed types is shown in their superior resistance to decay, and is thus of geolo- gical signifiance. The relations of anhydrous to hydrous species of aluraiuous double sili- cates appear to be of less importance, when we consider what secondary causes will deter- mine the formation either of a hydrous or an anhydrous species, of a zeolite or a feldspar.'"' The relations of the bases, potash, soda, and lime to each other, and to magnesia and other protoxyd-bases, are next to be considered, alike for the double aluminous silicates and for simple silicates of protoxyds. A system of classification, constructed in accordance with th. • principles, has already been indicated in the preceding illustrations of the crenitic hypothesis, and will, it i,'; believed, be found of fundamental importance for the student of mineral physiology ; since it is based on the genetic processes by which the species of the mineral world have in most cases been formed. The principles which it embodies, will be found not less applicable to compounds of igneous origin than to those formed by aqueous processes. § 109. In considering the origin of crystalline stratified rocks formed, in accordance with our hypothesis, in all cases with the concurrence of water, questions connected with the process of crystallization of mineral species, and of their condition when first deposited, are of much importance. The most familiar case is that of the direct separation of matters in a crystalline condition, as happens from the evaporation or the (change of temperature of the solvent, or from the generation of now and less soluble compounds, as in many cases of che- mical precipitation. In this connection, it should be noted " that many such compounds, when first generated by double decomposition in watery solutions, remain dissolved for a greater or less length of time before separating in an insoluble condition There is reason to believe that silicates of insoluble bases may assiime a similar state, and it will probably be showJi one day that for the greater number of those oxygenized com- pounds, which we call insoluble, there exists a modification soluble in water. lu this con- nection also may be recalled the great solubility in water of silicic, titanic, stannic, ferric, aluminic and chromic oxyds, when in what Graham has called the colloidal state." '" In writing the above, in 1874, reference was also made to my own earlier observ^ations on the solubility, under certain conditions, of carbonate of lime, which are subjoined. § 110. " The recent precipitate produced by a solution of carbonate of soda in chlorid of calcium is readily soluble in an excess of the latter salt, or in a solution of sulphate of magnesia. The transparent, almost gelatinous magma, which results when solutions of carbonate of soda and chlorid of calcium are first mingled, is immediately dissolved by a solution of sulphate of magnesia, and by operating with solutions of known strength [titrated solutions] it is easy to obtain transparent liquids holding in a litre, besides three 01 four hundredths of hydrated sulphate of magnesia, 0.80 gramme, and even 1.20 grammes, of carbonate of lime, together with 1.00 gramme of carbonate of magnesia; the only other substance present in the water being the chlorid of sodium equivalent to these car- '■^Soo, in this connoction, tlm author On tlio Ohjw-ta and Method of ^Mineralogy, Choni. and Geol. Essays, pp. 452-458 ; also the same, pp. 445-447. " On the relations of hydrous to anhydrous species, see farther the author in Trans. Roy. Soc. Can., vol. i., part 4, p. 208. "' Hunt, Chom. and Qeol. Essays, p. 223. OB[GIN OF CRYHTALLINE BOOKS. 83 the olo- ili- er- ar."" hor for has ill, ry; ive bonates. A solution of ohlorid of magiiosium, holdinjT somo chlorid of sodium aud sulphate of maguesia, in like manner dissolved 1 .00 gramme of carbonate of lime to the litre. Such solutions have an alkaline reaction." These solutions, which contained, in all cases, neutral carbonates, with no excess of carbonic acid, possessed a consideral)le degree of stability. One prepared with 0.80 gramme of carbonate of lime and 1.00 gramme of carbonate of magnesia, wheii filtered after standing eighteen hours at 10' C, still retained 0.72 gramme of carbonate of lime to the litre, but, after some days, deposited the whole of this in transparent crystals of hydrous carbonate of lime, all of the carbonate of magnesia remaining dissolved. This liydrous car- bonate, stable at low temperatures, is at once decomposed, with loss of its water, at 30' C. "The solubility of the yet uncondensed carbonate of lime in neutral solutions, which are without action upon it in another, state of aggregation, is a good example of the modilicd relations presented by bodies in the so-called nascent state, which probably, as in this case, consists of a simpler and less condensed molecule. At the same time, the gradual spontane- ous decomposition of the solutions thus obtained affords an instructive instance of the influence of time on chemica' channes.""- § 111. The spontaneous conversion of uncrystalline precipitates into crystalline aggre- gations may next be noticed. Instances of this are well known to chemists, but a remark- able and hitherto undescribed example is afforded in the case of the mixed oxalates of the cerium-metals, got by precipitating their nitrici solution with oxalic acid in the cold. A tough pitchy mass was thus repeatedly obtained which, in a few minutes, changed into incoherent crystalline grains, the conversion being attended with a notable evolution of heat. Another example of a somt^what similar phenomenon is presented in the case of the amorphous insoluble malate of lead, which, as is well known, spontaneously changes into crystals beneath the liquid in which it has been precipitated. § 112. In the paper above quoted on the salts of lime and maguesia, I have described not less remarkable examples of similar transformations in the case of the carbonates of lime aud magnesia. A paste of hydrous carbonate of magnesia precipitated in the cold, slowly changes under water, at ordinary temperatures, into a crystalline mass made up of prisms, grouped in spherical aggregations, of the well-known terhydrated magnesian carbo- nate. In like manner, the amorphous paste got by triturating in a mortar a solution ofchlorids of calcium aud magnesium, in equivalent proportions, with the requisite amount of a solution of neutral carbonate of soda is, at a temperature of from 65° to SC C, changed, after a few hours, into an aggregate of translucent crystalline spheres of a hydrous double carbonate, resembling the hydrodolomite of von Kobell. At temperatures of from 15' to 18' C, the same magma changes slowly into a more highly hydrated com- pound. The process of change, which requires from twelve to twenty-five days, appeared "to consist in the formation of nuclei from which crystallization proceeded until every particle of the once voluminous, opaque, and amorphous precipitate had become translucent, dense, and crystalline." The product is made up of brilliant prisms, apparently oblique, grouped around centres, and sometimes forming .spheres five or six millimetres in diameter. The hydrated double carbonate of lime and magnesia, thus formed in presence of a "'Hunt, Contributions to the History of Lime and Magnesia Salts; Part ii., 186C. Amer. Jour. Science, vol. Xlii., pp. 58, 50. 36 DR. THOMAS STERRY HUNT ON THR slight exocss of carbonate of soda, was found to contain more than two per cent, of the latter, but it was not certain whether this did not proceed from an admixture of the hydrous double carbonate of lime and soda, gaylussite. The new composition itself was descri^ .'d as having the composition of a gaylussite, in which magnesium replace sodium. The production of crystals of true guylussite, as observed by Fritzsche, by the slow crystalliza- tion of the gelatinouis precipitate got when a strong solution of carbonate of soda in excess is mingled with one of calcium-chlorid, is another remarkable example of the phenomenon under consideration. Fritzsche moreover observed that it is not necessary that the lime-carbonate should be in its gelatinous form in order to produce this compound, since the previously precipitated carbonate when digested with a solution of carbonate of soda, slowly combines with it to form the crystalline hydrous double salt. More remarkable still is the observation of H. Ste. Claire Deville, which I have repeatedly verified, that a paste of magnesia alba and bicarbonate of soda, with water, is slowly changed, at a temperature of from 60° to *70 ' C, into a transparent crystalline anhydrous double carbonate of lime and soda, hexagonal in form, and called by its discoverer a soda-dolomite.''' § 113. In this connection, it should be said that we have here an explanation of the formation of the double carbonate of lime and magnesia which constitutes ordinary dolo- mite. The origin of this mineral species, which so often constitutes rock-masses, is still generally misunderstood. The baseless notion of its production by a metasomatosis or partial replacement of the lime in ordinary limestone, imagined by the older geologists, is still repeated, and holds its place in the literature of the science, despite the facts of geog- nosy and of chemistry. I have long since shown, by multiplied examples, that the ordinary mode of the occurrence of dolomite in nature is not in accordance with this hypothesis of its origin, since beds of dolomite, or more or less magnesian limestone, are found alternating, sometimes in thin and repeated layers, with beds of non-magnesian carbonate of lime. Moreover, beds of crystalline dolomite, conglomerate in character, are found to enclose pebbles and fragments of pure non-magnesian carbonate of lime. I have also ex- plained at length the natural reactions by which prei-ipitates consisting of a greater or less proportion of hydrous carbonate of magnesia, mixed with carbonate of lime, must, in past ages, have been laid down in the waters of lakes and inland seas, in some cases with, and in others without, the simultaneous formation of sulphate of lime. It was, moreover, found that the reaction at an elevated temperature in presence of water, between sulphate of magnesia and an excess of carbonate of lime, supposed by Haidinger and von Morlot to explain the frequent association of gypsum and dolomite, does not yield the double carbonate, since the carbonate of magnesia separates in an anhy- drous form, and does not combine with the carbonate of lime. Finally, it was shown that mixtures of hydrous carbonate of magnesia and carbonate of lime, when heated together in presence of water, unite to form the anhydrous double carbonate which constitutes dolo- mite. In my experiments, their combination, with the formation of dolomite, was effected rapidly, at 120° C, but many considerations lead to the conclusion that its production in "Hunt, Contributions to the History of Lime and Magnesia Salts, part ii., 1866. Amer. Jour. Science, vol. xlii., pp. 54-57. '■•"^'mmmmm S W i' lwJ.M.M i m i- ORIGIN OP CIIYSTALLINI'] KOtUvS. 37 atter, rous The nature is effected slowly at much lower tempenituivs, and that the formation of the hydrous double carbonate already described is, perhaps, au intermediate stage iu the process."" § 114. The reactions described in the preceding paragraphs between the elements of comparatively insoluble substances in the presence of water, resulting not only in the con- version of amorphous into crystalline bodies, but in the breaking-up of old comljinatious, as well as iu the union of unlike matters mechanically mingled to form new crystalline species, are instrucitive examples of what Gihnbel has termed diaoknesls. The changes in the masonry of the old llomau baths in contact with thermal waters, resulting in the hydration of the substance of the bricks, and its conversion into zeolilic minerals; tiie hydration of volcanic glasses, with similar results, going on, even at low terapi-raturcs, in the deep sea ; the decomposition of common glass by heated water ; the conversion of basaltic rock into palagonite and the production therefrom of zeolites ; the similar changes seen elsewhere in amygdaloids, and even in massive basii- plutonic rocks, are also examples of this process of diagenesis, and serve to show its great geological significance. We have already suggested the intervention of similar reactions in past ages among the sediments from the subacrial decay of felspathic rocks, in some cases with the concurrence of the secretions from the primary basic stratum, which, in accordance with the crenitic hypothesis, we suppose to have been the sources of solul)le mineral silicates. In the diagenesis of these early argillareoxis sediments, aided by creniti(; action, will, it is believed, be found the origin of many of the crystalline schists of the transition rocks. § 115. An instructive phase in this diagenetic process is that of the gradual conversion of smaller crystalline grains or crystals into larger ones, which is familiar to chemists. This action is in fact nearly akin to that which takes place in the transformation of amor- phous into crystalline precipitates, since in both cases a partial solution precedes the crystallization. It is well known that, as a result of successive solution and re-deposition, large crystals may be built up at the expense of smaller ones. This process, as H. Deville has shown, " suffices, under the influence of the changing temperature of the seasons, to convert many fine precipitates into crystalline aggregates, by the aid of liquids of slight solvent powers. A similar agency may be supjiosed to have effected the crystallization of buried sediments, and changes in the solvent power of the permeating water might be due either to variations of temperature or of pressure. Simultaneously with this process, one of chemical union of heterogeneous elements may go on, and in this way, for example, we may suppose that the carbonates of lime and magnesia become united to form dolomite or magnesian limestone." "" § 116. The tendency of the dissolved material hi this process to crystallize around nuclei of its own kind, rather than on foreign particles, is a familiar fact, and its geological import- ance, to which I first called attention, as above, in 18G0, was again pointed out by Sorby in 1880, when he showed that dissolved quartz might be deposited upon clastic grains of this mineral in perfect optical and crystallographic continuity, so that each broken frag- ment of quartz is changed into a definite crystal, as was seen in his microscopic '""Hunt, Contributions to tlio f'liomistry of Linio ami JNIagnosia, part i., 1859. Anior. .Tour. Sci., xxviii., pp. 170,365, and part ii., ISOO, vol. xii., p. 40; alao in alwtract in Ciioni. and tlcol. Essays, pp. 80-i)2. "" Hunt, Tho Chemistry of tlio Kartli, Rojxirt Sniitlisonian Institution, 1869 ; also Chom. and Gool. Essays, p. 305. Sec. III., 1884. 8. 88 DR. THOMAS ST EERY HUNT ON THl'l ri> studies of various sandstones.'"^ This fact has been confirmed by the observations of Young, Irving, and Wadsworth in the United States ; "" and Bonney has suggested the possible extension of such a process to feldspar, hornblende and other minerals."" Vanhise has very recently announced that his microscopical examinations of cer- tain sandstones of the Kewcenian series, from Lake Superior, afford evidence of tht! second- ary deposition of both orthoclase and plagioclase feldspar, in crystallographic continuity upon broken feldspathic grains, in one case uniting the two parts of a broken feldspar- crystal. The sandstones which have yielded these examples are made up in part of feld- spathic fragment.s, and in part of fragments of "some altered basic rocks." They are more- over, iuterstratified with and, in some case at least, immediately underlie the basic plutonic rocks of the same Kewcenian series. "'^'' When we consider that orthoclase is a common secretion of these basic rocks, as is shown by its frequent occurrence in them with zeolites and epidote, it may perhaps be questioned whether the secondary feldspar in the sand- stone has been derived from the adjacent grains of this mineral, or has come into solution from the transformation of the Ijasic rocks. The apparent stability and insolubility of orthoclase and oligoclase at high temperatures in the presence of water, as observed by Daubree, would seem to favour the latter view. In any case, it is a striking illustration of the tendency of mineral species to crystallize around nuclei of their own kind, which is so marked a factor in the development of the crystalline rocks. IV. — Conclusions. § 111. "We reviewed in the first part of this essay the history of the different hypotheses hitherto proposed to explain the origin of the crystalline rocks and, in doing so, reached the conclusion that not one of them affords an adequate solution of the various problems pre- sented by the chemical, mineralogical and geognostical characters of the rocks in question : at the same time, we endeavored to show succinctly what are the principal cciiditions to which a satisfactory hypothesis must conform. In the second part, we sketched the growth and development, during the last quarter of a century, of what we believe to be such a hypothesis. In the third part, we sought to bring together a groat number of facts, both new and old, which serve to illustrate the new hypothesis ; according to which the crystal- line stratiform rocks, as well as many erupted rocks, are supposed lO have been derived by the action of waters IVom a primary superficial layer, regarded as the last portion of the globe solidified in cooling from a state of igneous fluidity. This, which we have described as a basic, quartzless rock, is conceived to have been fissured and rendered porous during crystallization and refrigeration, and thus rendered permeable to considerable depths to the waters subsequently precipitated upon it. Its surface being cooled by radiation, while its base reposed upon a heated solid interior, upward and downward currents would establish a system of aqueous circulation in the mass, to which its porous but unstratified condition would be very favorable. The materials which heated subterraneous waters would bring '^^ Soriiy, Presidential Address, Quar. .Tour. Goo. Soc. London, xxxvi., 33. "» Young, Amer. Jour. Sci., xxiv., 47. Irving, Ibid, xxv., 401. Wadsworth, Proc. Boston Soc. Natural History, Feb. 7, 1883. '"* Bonney, Quar. Jour. Geol. Soc. xxxix., 10. '»» Vaahise, Amer. Jour. Sui., 1884., xxvii., 399. mmm ORIGIN OF CRYSTALLINE ROCKS. 89 ble ]d- to the surface, there to be deposited, would be not unlike those which have been removed, by infiltrating waters in various subsequent geological ages, from erupted masses of similar basic rock ; which, we have reason to believe, are but displaced portions of this same primary layer. The mineral species removed from these latter rocks, or segregated in their cavities, are, as is well known, chiefly silica in the form of quartz, silicates of lime and alkalies, and certain double silicates of these bases with alumina, including zeolites and feldspars, besides oxyds of iron and carbonate of lime ; the latter spoci(\s being due to the intervention of atmospheric carbonic acid. The absence from those minerals of any considerable propor- tion of iron-silicate, and, save in rare and exceptional conditions, of magnesia, is a significant fa(;t in the history of the secretions from basic rocks, the transformation of which, under the action of permeating waters, has resulted in the conversion of the material into quartz and various silicates of alumina, lime, and alkalies, while leaving behind a more basic and insoluble residue abounding in silicated compounds of magnesia and iron-oxyd with alumina. § 118. The peculiarities resulting from this comparative insolubility of magnesian silicates long ago attracted the attention of the writer. The addition, to solutions like sea- water, of bicarbonate of magnesia, which is a product of the sub-aerial decay of basic rocks, would, it was shown, effect a separation of dissolved lime-salts in the form of carbonate, leaving the magnesia in solution as chlorid or as sulphate ; while on the contrary the action of such a natural water with certain silicates, whether solid or in solution, containing lime or alkalies, would effect a removal of the dissolved magnesia. At the same time it was shown that, " by digestion at ordinary temperatures, with an excess of freshly precipitated silicate of lime, chlorid of magnesium is completely decomposed, an insohible silicate of mag- nesia being formed, while nothing but chlorid of calcnum remains in solution. It is clear that the greater insolubility of the magnesian silicate, as compared with silicate of lime, determines a reaction the very reverse of that produced by carbonates with solutions of the two earthy bases. In the one case, the lime is separated as carbonate, the magnesia remain- ing in solution, while in the other, by the action of silicate of soda, or of lime, the magnesia is removed and the lime remains. Hence carbonate of lime and silicate of magnesia are found abundantly in nature, while carbonate of magnesia and silicate of lime are produced only under local and exceptional circumstances. It is evident that the production from the waters of the early seas of beds of sepiolite, talc, serpentine, and other rocks in which a magnesian silicate abounds, must, in closed basins, have given rise to waters in which chlorid of calcium would predominate." '* § 119. From this reaction it would follow that the magnesian salts, formed when the first acid waters from the atmosphere fell upon the primary stratum, would be removed from solution, either by the direct action of the solid rock, or by tliat of the pectolitic secretions derived therefrom in the earliest ages. The primeval ocean, if, as we suf*- pose, a universal one, would soon bo deprived of magnesian salts, and henceforth the early-deposited rocks would be essentially granitic in composition, a nd uon-magnesian, until the introduction of magnesia into its waters from an exterior source. The pectolitic silicates, themselves, which, in the cavities of exotic basic rocks, are deposited in crystalline forms, would, if set free in a Sea deprived of magnesian salts, be "" Amer. Jour. Sci., 18C5, vol. xl., p. 49 ; also Chem. and Gool. Essays, p. 122. 60 DE. TnOMAS STERRY HUNT ON THE ■ii readily decomposed by the carbonic acid everywhere present, with separation of free silica and carbonate of lime. From this would be formed the lirst deposits of limestone, which make their appearance in the old gneissic rocks and become mingled with magnesian car- bonate and silicates from the introduction of magnesian salts into the waters. The comparative instability of the linn'-silicate is seen when woUastonite is compared with the corresponding silicates, pyroxene and eustatite. It is possible, notwithstanding the absence of magnesian species from zcolitic secretions, that, under certain conditions, small portions of magnesian silicate may have been included in the early crenitic deposits, but the rarity of such magnesian silicates in these, and their abundance in parts of the later Laurentian and in younger deposits, points to a new source of the magnesian element ; namely, the extravasation of portions of the underlying primary mass, and its sub- aerial decay. It woiild be instructive to consider in this relation the gradual removal of a large pro- portion of silica from the primary stratixm in the forms of orthoclase, albite and quartz, and the consequent partial exhaustion of portions of this underlying mass, so that its suc- ceeding secretions consisted chielly of less silicic silicates, such as labradorite and andesite, without quartz, as in the Noriau series. § 120. The conditions of this first exoplutonic action cannot be fully understood nntil we have settled the question of the permanence of continental and oceanic areas, and the extent of the early crenitic rocks which constitute the fundamental granites and the granit- oid gneisses. Whether these are spread, with their vast thickness, alike underneath the great areas of the paleozoic series and our modern oceanic basins ; in brief, whether or not they are universal, as sui^posed by Werner, is a question which cannot here be discussed. There is, however, nothing incompatible with what we know of the chemistry of the early rocks and the early ocean in the supposition that they were universal, since there is apparently no evidence that the products of subaiirial decay of exposed rocks intervened in their production. Such a condition of things was, however, necessarily self-limited ; the great diminution of the primary mass, from the constant removal of portions of it in a state of solution, and the vast accumulated weight of the superincumbent accumulated granitic and gneissic material, could not fail to result in widely spread and repeated corrugations and foldings of the overlying mass, the e fleets of which are seen in the universally wrinkled and frequently vertical attitude of the oldest gneissic rocks. Such a process, like the simi- lar though less considerable movements in later times, would probably be attended with outflows, in the form of fissnre-eruplions, of the underlying basic stratum, which, in accordance with our hypothesis, was permeated with water under conditions of tempera- tures and pressure that must have given to it a partial liqiiidity. Such a process of collapse and corrugation of the crenitic deposits, attended with extravasation of the underlying primary stratum, would, doubtless, be often repeated in these early periods, resulting in frecjuent stratigraphical discordances, which are, however, in all cases to be looked upon as local accidents, and not as wide-spread catastrophes. Hence the appearance, from time to time, of exoplutonic rocks, with upliftings and depressions of the older rocks, which caused the exposure of both alike to the action of the atmosphere. § 121. The consequent subaerial decay of these two types henceforth introduced new factors into the rock-forming processes of the time, and made the beginning of what Werner called the Transition period. The decomposition of these, under the influence of a moist ■I ORIGIN OF CRYSTALLINE ROCKS. 61 atmosphere holding carbonic acid, resulted in the more or less complete removal of the alkali from the feldspars of crenitic rocks, and their conversion into kaolin, while the corres- ponding <'hangi>8 in the basic exoplutonic rocks were still more noteworthy. These rocks, while containing feldspars, consisted in large part of silicates of lime and magnesia, pre- sumably pyroxene and chrysolite, which, as wo an; aware, yicild to the action of the atmos- phere the whole of their lime and magnesia. Thes(>, in the form of carbonates, passed into solution together with a large proportion of silica, leaving behind the remaining portion, together with non-oxyd and the kaolin from the feldspars. The carbonates of alkalies, of lime, and of magnesia, resulting from the subaerial decay of the exposed exoplutonic and the crenitic rocks alike, were carried to the sea, there to play an important part. Besides the direct influx of carbonate of lime into the waters of that time, it is evident that both the alkaline and the magnesian bicarbonates would rea<-t upon the calcium-chlorid of the primeval sea, with the production of a farther amount of lirae-carbouate, and the generation of alkaline and magnesian chlorids. In this way, the sea becoming magne- sian, a new order of things was established. Henceforth, the pectolitic matters brought up from the primary layer would at once react upon the dissolved magnesian salts, and the production of such compounds as chondrodite, chrysolite, serpentine, and talc would commence. No one who has studied the mode of occurrenc(> of these silicates in the upper part of the Laurentian series, where serpentine not only forms layers, but frequent concre- tions like flints, often around nuclei of white pyroxene, can fail to recognize the process which then came into play, resulting later in the production of abundance of pyroxene, hornblende and enstatite, and apparently reaching its culmination in the vast amount of magnesian silicates found in the deposits of the Huronian age. § 122 The solutions of simple silicates of alkalies, which by heat had deposited their excess of silic^a in the form of quartz, as in the case of the soluble matter from glass, probably gave rise by their reaction with magnesian solutions to the basic protoxyd-silicates, like chondrodite, chrysolite, serpentine and pyroxene. That we have no anhydrous quadri- silicates corresponding to apophyllite and okenite is apparently due to the fact that such silicates, in contact with water at elevated temperatures, break up into anhydrous bisili- cates and quartz; as is seen in the artiiicial association of pyroxene and quartz in the experi- ments of Daubree, and the frequent occurrence of admixtures of the two in beds among the ancient gneissic rocks. A noticeable fact in the history of the surbasic silicates of mag- nesia and related protoxyd-bases, mentioned above, is their frequent association with non- silica ted oxyds. Examples of this familiar to mincn-alogists are the occurrence of .aggregates of chondrodite and magnetite ; of chromite, picotite, ilmenite and corundum with chry- solite and serpentine ; and of franklinite and zincite with tephroite and willemite. These collocations are probably connected with the solvent power of solutions of alkaline silicates, already insisted upon (§ 89), and probably also with the dissociation of silicate of alumina in heated alkaline solutions, noticed by H. Deville {§ 98). The separation, by the alternate action of decaying organic matters and of atmospheric oxygen, of iron-oxyd, which readily passes from a soluble ferrous to an insoluble ferric condition, and conversely, has probably played an important part in the formation of depo- sits of iron-oxyds, which are much more cosmopolitan in their associations than corundum, or the compounds of chromic, titanic, aluminic, manganic and zincic oxyds mentioned above, to which we have assigned a diiFerent origin. It will remain for the mineralogist 62 DR. THOMAS STERTIY HUNT ON THE to (letermiuo what deposits of magnetite and of hematite are to be ascribed to the oue and what to the other origin. § 123. We have seen, among the secretions of basic rocks, lime-alumina silicates, like cpidote and prohnito, in which tho ratio of the protoxyd-bases to alumina, instead of being 1 : .'5, us in the feldspars and the zeolites, is 1.J : :$ or oven 2 : 3. Although, probably on account of their solubility and their instability, we do not know of any natural sili- cates with a still larger proportion of lime to the alumina, we have indirect evidence of their former existence in solution, in the frequent occurrence of double silicates of magnesia and alumina, in which the oxygen-ratio of 1{, al, instead of being 1 : 3, as in the feldspars, or 2 : ;?, as in prehnite, becomes 3 : 3 and even atible with the existence of organic life. §127. The phenomena of exoplulouic action, or so-called vulcanicity, though relegated to a secondary i>lace in ihi^ crenilii' hypothesis, are yet, ivs we have said, of great importance and signilicance, and are by no means sim[)le. They were, according to our hypothesis, conlined in early times to iissure-eruptions of the underlying primary stratum. This, although in the course of ages it ha.s sull'ered a gradual change from the ceiuseless crenitic action, which has removed from il the elements of the various series of crystalline rocks, including the primitive granitic and gneissic series, probably still retains in the lower portions soujewhat of its original constitution. A second phase in the history of exoplu- toiiic rocks, air Midy foreseen by the Iluttonians, here presents itself for our consideration. The more deeply buried portions of the primitive crenitic deposit, must themselves have boon brought within the inlhu'uce of the central heat, and, permeated as they were by water, have sull'ered a softening which permitted them, as a result of subsequiMit move- ments of the crust, to appear again at the earth's surface as exoplutonic or exotic rocks of the trachytic' or granitic type. AVe can hardly supposi' the displacement, either of the primary igneous mass or of the early granitic deposits, to have lu-en attended with the evolution of pi'rmanent gases, such as attend modern volcanic eruptions and are to be ascribed to the action of subterraneous heat on more recent deposits, including carl)onates, sulphates, chlorids and organic matters. Such materials, when mingled with siliciims and argillaceous sediments, a'.id brought by local accumulation and tlei)ression within the heated zone would give rise to the various gases which iharacterize th(> volcanic eruptions of recent periods, in which, however, the materials of the xmderlying primary and (Touitic layers apparently intervene. By thus ascribing a three-fold origin to the products of exoplutonic action, it becomes possible to classify and liarmoniz(> the api>an'ntly discordant i)hi'nomena of eruptive ro( ks. While the typical ba.salts and related basic rocks would be derived from the primary igneous stratum, and the trachytic and granitic rocks from the earlier crenitic, deposits, the more fusible portions of the later transition and secondary strata may have furnished their contingent, not only of gases and vapors, but of lavas and volcanic dust. § 128. The history of the origin of crystalline rocks is the history of the origin of the mineral .species which compose them. I'he crystalline masses are essentially made up of a few groups of species. Various feldsi)ars and occasional zeolites, some of which apparently occur as integral parts of rocks chiefly feldspathic, form a great central group. On OIK! side of these are the aluminous d()u])le silicates, represented by basic species like garnet, epidote, magnosian micas and chlorites, all with an excess of protoxyd-bases ; while, oil the other hand are the aluminous double silicates of the muscovitii- and pinitic groups, in which the diminished proportion of the protoxyd-bascs prepares the way to the associated simple aluminous silicates, pyrophyllite, andalusite, cyanite, etc To theso ORIGIN OF (CRYSTALLINE ROCKS. 68 groups must bo acldod the nou-aluminous silicates, including honiblcudo, pyroxeuo, eusta- tite aud chrysolite, and tho hydrous maguesiaii species, serpentine and tahi. Besides these are free silica, generally as quartz. Tree oxyds, iutluding the spinel and corundum groups, which, together with tho carbonates, make up tho essential parts ofihe crystalline rocks. § 121). Jiock-masscs, and tho mineral speiiies whiih compose Iheni, present variations in time, as we find in triiving tho history ol" tho great successive groups of crystalline strata; and they moreover show local changes, as seen in dillereut parts of their distribution in tho same geological group. As regards the causes ot these variations, very much remains to bo discovered by Iho patient collection and recording of facts concerning the associations of mineral species, their artilicial production, and their transformations under tho inlluences of firo aud water, and of solutions of potassio, sodic, calcareous and magnosian salts. Tho instability of silicated compounds of igneous origin in tho presence of water and watery solutions, so widely dill'used through nature, i.s tho warrant for a general aqueous hypo- thesis ; while, on tlio other hand, Iho deriA'ation of stable mineral species, under such inlluences, from matters of igneous origin justilies us in assuming for these species an igneous starting point. Igneous fusion destroys the mineral species of the crystalline stratified roiks, and brings them back as nearly as possible to tho i)rimary undifFcrentialod material. Fire is the great destroyer and disorganizer of mineral as well as of organic matter. 8ubtorra- noau heat in our lim •, acting upon buried a(|ueous sediments, destroys carboMutes, sul- phates, and chlorido-., with tho evolution of acid gases and tho generation of basic silicates, and thus repeats in miniature the conditions of the anio-neptunian chaos, with its surroundii 'S; n. On the otiier hand, each mass of cooling igneous rock in contact v, .'tii water begins anew tin; formative process. The hydrated amorphous product, palagonito, is, if we may bo allowed the expression, a sort of silicated protoplasm, N^hich, in its differentiation, yields to tin; solvent action of water tho crystalline silicates which are tho constituent elements of tho crenitic rocks, leaving, at the same time, a more basic residuum abounding in magnesia and iron-oxyd, and soluble not by crenitic but l)y sub-aerial action, ralagonite, or some amori)hous matter resembling it, probably marks a stage in the sub-aqueous transformation of all igneous rocks, though only Under special conditions does this unstable, hydnms substance form appreciable masses. In all cases, igneous matter, of primary or of secondary origin, serves as the point of departure. According to tho proposed hyi)otIiesis, which derives rocks of the granitic type, com- posed essentially of quartz and feldspars, by aqueous secretion from a primary igneous aud quartzless mass, it would follow that the highly basic compound, assumed by Bunseu to represent the typical pyroxi'uic or basaltic rock (§ 24), would be the above mentioned insoluble residuum; and that less basic varieties of similar rocks would correspond to portions of tht>. same primary mass, less completely exhausted by lixivialion, and conse- quently approaching in composition to admixtures of tho basaltic and granitic types, as maintained on other grounds by Bunsen himself. § 130. Tho principles whi.h have bo(>n enumerated in tho preceding pages will, it is believed, load tho way, not only to a natural systitm of mineralogy, but to a natural system of classification of crystalline rocks, considered with regard alike to their chemical compo- sition, their genesis, aud their geological suocessiou. A valid hypothesis for the crystallmo Sec. in., 1884. 9. 66 DE. THOMAS STERRY HUNT ON THE rocks must seek to connect all the known facts of their history, by alleging a true and sufficient cause for the production of their various constituent mineral species. Such a hypothesis will violate no established principles in chemistry or in physics, but will show itself to be in accord with them all, and Mnll commend itself to the acceptance of those who take the pains to understand it. The crenitic hypothesis set forth in these pages is the result of many years of patient study applied to the elucidation of a great problem ; and as such is offered to chemists and mineralogists as a first attempt at a rational explanation of the fundamental questions presented by the history of the crystalline rocks of the earth's crust. • •■ ■ Contents OF Sections. • . . ■ I. — Hulorkal and Critical. — § 1, 2. Supposed igneous origin of crystallino rocks ; views of Lehman, Pallas, de Luc and Saussuro.— 3, 4. Wornor's noptuniaa system ; liia primitive, transition and secondary rocks. — 5. System of Hutton; liis intorprotor Play fair; nature of granite. — 0, 7. Indigenous, exotic and endogenous rocks; significance of eruption in geology. — 8,9. The schools of Werner and Hutton contrasted; their unlike views of the origin of "ranit*^ and of gneis.s; Hutton the founder of the metamorphic scliool. — 10, 11. Button's system farther defined; its analysis by Daubr&i. — 12. The theological aspects of the two systems. — 13- Delaboche's modified neptunian system. — 14, 15. Daubree's later statement of the same ; the intervention of internal lieat. — Ifi. The granitic substratum of Werner adopted by Hattonians. — 17. Poulett Scrope's theory of the origin of granite and of gneiss. — 18. Beroldingen and Saussure on the dotrital origin of gneiss; views of Bouij. — 19. Lyell on the Huttonian or metamorphic hypothesis; Bischof and Haidingor on psoudomorphic alteration ; a meta.'somatic hypothesis. — 20. Naumann's criti- cism of motamorphiam ; the chaotic, endoj)lntonic, exoplutonic and thermochaotic hypotlieses. — 21,22. - '. Endoplutonism as defined by Nauinann and liy II<5l»rt; Macfarlane's statement of the endoplutonic ; and thermochaotic hyiwthesos. — 24. Supposed condition of the earth's interior ; the two magmas of Bun- sen ; Von Waltershauson's views.— 25. The exoplutonic or volcanic hyiwthesis as stated by J. I). Dana in 1843. — 26. Metamorphism by an incandescent ocean. — 27, 28. Tliis view since abandoned by Dana ; , . his statement of the metamorphic hypothesis.— 29. Clarence King on metamorphism, and the sup- ^_ . jMsed igneous origin of olivine. — 30. Kopp, T()rnebohm and Reusch on the exoplutonic hypothesis. — • _ 31. Marr and C. H. Hitchcock on the same ; its relation to the origin and permanence of continents. — 32. Viirious geologist.s on the supposed eruption of limestones, Korjxsntines and iron-ores ; H. D. Rogers and Belt on tlie ignoou.s origin of quartz lodes. — 33. The eruptivti origin of rock-salt, buhrstone and cer- tain clays and sands maintained by many.— 34. Water in the formation of granites ; Scrojxi, i,\i& do Beaumon*, andSchcorer; pyrognomic minerals; hydroplutonic origin of 8eri)entine.— 35. Excesses of iho exoplutonic school ; transmutation or metasomatism.— 30. Metasomatosis of silicatod rocks ; their supposed conversion into serpentine and limestone; King and Rownoy.— 37, 38. Chrysohte rocks of ; ,' tile Atlantic belt; conflicting views of Genth and Julien as to their supposed alteration (Dana's criti- cisms, foot-note).— 39, 40. Metasomatosis of limestones; views of Volger, Bischof and Pumpelly.— 41. , The endoplutonic, exoplr onic, metamorphic, metasomatic, chaotic and thermocriiaotic liypotheses summed up.— 42. The endoplutonic and exoplutonic reviewed. — 43, 44. The metamorphic, metasomatic and chaotic reviewed. — 15. The thermochaotic; conditions of tho problem of rock-formation. IL— 37i« Development of a New Hypolheria.—^ 47. The lines of investigation followed. — 48, 49. Order and succession of crystallino rocks.— 50, 51. Tho hypothesis proposed in 1858 of a solid globe and a superficial layer as the source of all rocks.— 62. Farther bjwculations in 1859 on the source of acidic and basic rocks.— 54. Developments in 1807-1' 'he secondary origin of granite, and the underlying primary basic stratum.— 55. The aqueous origin of many mineral silicates.— 50, 57. Geological significance of zeolites, and their relations to feldspars.— 58, 59. On feldspathic and granitic veins.— 00, 01. Relations of granitic veins to indigenous granites and gneisses.— 02. ronclusions announced iu 1879.— 03. Relations of alumina in silicates to protoxyd bases.— 04. Source «1 granitic elements ; Scht«irer, Elie do Beaumont.— <35. The secondary origin of granite.— 06. Bunsen on palagonite, its origin and changes ; composition of trachytic / OEIGIN OF CRYSTALLINE EOCKS. 67 and basaltic magmas and of palagonito (foot-note).— 67, 68. A primary quartzless roclc the source of granite and of crystalline schists. — 69. Its separation by water into a lower basic, and an upper acidic portion. — 70. Shrinking of the former and wrinkling of the latter; exoplutonic rocks; the crenitic hypo- thesis.— 71. Its growth from 1858 to 1884. 111.— Illustrations of the Crenitic Ilypothexis.—'^ 72- Tlie now hy|x)thosis doflnod.— 73, 74. The study of zeolitic rocks. —75. Zoolitt>s of Table Mountain, Colorado.— 76. Zeolites and foldspathic veins of Mount Royal.— 77. The mineral secretions of basic rocks classified. — 78. Zeolites and related silicates ; a tabular view. — 79. The thomsonite and nepholito series. — 80. Bar.sowitt*, iolito, and relato.l spocii's ; tlio natrolito and labrado- rite series; the faujasite and houlandito series; orilioclase. — 81. Prehnite and chlorastrolito; epidote, saussurite and meionite. — 82. Tiio protoxyd ba.ses of zeolites; hematite and magnetite; the poctolitic group and its related sixjcies; chondrodite, chrysolite, sor^xsntino, dewoylite; tabular view. — 84. The bisilicates, hydrous and anhydrous. — 85. Quadrisilicates.— 86, 87. Daubrde on the action of hot water on glass ; formation of quartz, diopsido, a ixsctolitic si«cios, and a soluble fsilicato of alumina and soda. — 88. His farther studies of feldspars, etc. ; Fremy on alkaline silicates. — 89. Ordway on alkaline silicates ; ' . solutions of water-glass dissolve metallic oxyds. — 90, 91. Tlio thermal waters of Plombiiiros ; recent formation of /.oolitic and iKictolitic silicates, quartz and calcite. — 92. Tlio alteration of bricks by these warm waters. — 93. Similar studies at other thermal springs. — 94. Recent production of zeolites in basalt " .' and sandstone. — 9.5. Their formation in doei>sea oo/.e. — 96,97. The production of zeolitic silicates in the laboratory ; results of Berzelius, Anmion, and Way. — 98. H, Deville on the formation of zeolites and quartz. — 99. Friodel and Sarrusin on the protl action of both zeolites auii feldspars in heated solutions. ■ — 100. Aluminous silicates with excess of protoxyd b.ises; their genesis. — 101. Reactions of zeolites with magnesian solutions; Way, Eichliorn, Bunsen. — 102. Aluminous silicates with deficiency of protoxyds, and simple aluminous silicates; muscovitic micas. — 103. Probable origin of those silicates. — 104. Pinite and related siwcies ; their couiposition. — 105. Their iuiixjrtance in nature ; the survival of the fittest. — ICKi. The supposed chemical relations of jjinito- — 107, 108. A natural system of mineralogical classi- fication. — 109. Conditions of crystallization from water; solubility of silicates and oxyds. — 110. TemiK)- rary solubility of carbonate of lime; a liydrocarbonate. — HI. Conversion of amorplious into crystalline matters; cerium-oxalates ; malato of lead.— 112. Crystallization of hydrous magnesian carbonate, and hydrous double carl)onates of lime and magnesia ; gaylussite ; tlie soda-dolomite of II. Deville. — 113. Dolomite, its geognostical and chemical history ; its origin and formation explainod.—114. Tlio process of diagenesis. — 115. Conversion of smaller into larger crystals; chemical union. — 116. Crystallization of dissolved matters around nuclei ; examples from quartz and orthoclase. IV. — Conclusions. — §117. The crenitic hypothesis re-stated. — 118 Separation of magnesian salts from sea-water ; the relations of carbonates and silicates of lime and magnesia. — 119. Elimination of magnesia from the primeval sea ; decomposition of poctolitic silicatea by carbonic acid, and formation of limestone. — 120. * First exoplutonic action ; results of contraction of the primary mass. — 121. Sub-aiirial decay of exotic rocks a source of magnesian salts ; tlieir reaction with poctolitic silicates. — 122. Decomposition of alka- line silicates ; formation of quartz and basic silicates ; oxyds of the spinel and corundum groups ; their formation. — 123. Surbasic aluminous silicates ; rocks of the Huronian time. — 124. Absenco-of magnesia from sea-basins ; Montalban gneisses and mica-schists ; sub-aiirial decay ; boulders of decomposition ; iolite-gneiss.— 125. Taconian Bories; its micaceous schists; Laurentian, Arvonian, Norian, Huronian, Montalban and Taconian series; tlieir relations noted. — 126. Later evidences of crenitic action; Silurian serpentine and tertiary sepiolito; conditions of deposition of aluminous silicates compatible with life. — 127. Exoplutonic or volcanic phonomena; their probable three-fold origin.— 128. The history of crystal- line rocks resumed. — 129. Their changes in time and place; grounds of an aqueous hypothesis and an igneous point of departure ; fire as a destroyer, water as an organizer ; relations of palagonito ; separa- tion of the primary mass into basic and acidic layers.— 130. Marks of a valid hypothesis; claims of the one here proposed. Note. — The observations of Vanhise, cited in § 116, have appeared since the presenta- tion of this paper in May, 1884. The same is true of those of Murray and E6nard, referred to in § 95, though these had previously been communicated to the present writer.