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LL.D., F.R.S. i I (Read before the American Association for the Advancement of .Science, at Saratoga. t?eptember 1. 1879.) I. Introduction. One of the earliest distinctions in modern geology was that between the crystalline or so-called Primary .strata, and tho.se which are found in many cases to have been deposited upoD them, and being in part made up nf .sediments derived from the disintep ration of tliese, were designated Transition and Secondary rocks. While the past forty years have seen great progress in our knowledge of these younger rocks, and while their strati- graphy, the conditions of their dposition, and their geographical distribution and variations have been carefully investigated, the btudy of the older rocks has been comparatively neglected. This has been due in part to the inherent difficulties of the subject, arising from the general absence of organic remains, and from the highly disturbed conditimi of the older strata, but in a greater measure, perhaps, to certain theoretical views respecting the stratified cry.stalline rocks. In fact, the unlike teaching.'^ of t\vo different and opposed schools lead to the couunou conclusion that the geognostical study of these rocks is unprofitable. The first of these schools maintains that the rocks in question are, in great part at least, not subordinated to the same structural laws as the uncrystallinc formations, but are portions of the original crust of the earth, and that their architecture is due not to aqueous deposition and subsequent mechanical movements. [ HUNT — PRE-CAMBRIAN ROCKS. hut rnthor to agencies at work in a cooling igneous mass The ignoous origiii of gneisses, petrosilex-porphyries, diorites, ser- pentines, and even of magnetic and specular iron ores was held and taught almost universally by our geologists a generation since, and has still its avowed partizans; some maintaining that these various crystalline rocks arc portions of the first-formed crust of the planet, while others imagine them to be volcanic matters extravasated at more recent date ; in either case however, more or less modified ' by supposed metasomatic processes. By the term metasomatosis are cmiveniently designated those changes which are not simply internal (diagenesis), but are efi'ected from without. — as a result ol" which the chemical elements of the original rock are supposed to be either wholly or in part replaced by others from external sources (epigenesis). The other school, to which allusion has been made, and which, not less than the preceding, has helped to discourage, in the writer's opinion, the intelligent geognostical study of the crystal- line stratiform rocks, is that which believes them to be, in great part at least, the result of chemical changes, often metasomatic in their nature, which have been eifected in paleozoic and more recent sedimentary beds, obliterating their organic remains, and transforming them into crystalline strata. According to this view, feldspathic, horiiblendic, and micaceous stratiform crystal- line rocks having similar mineralogical and lithological characters, may belong to widely separated geological periods, — while the same geological series may, in one part of its distribution, consist of uncrystalline silicious, calcareous, and argillaceous fossiliferous sediments, and in another locality, not far remote, be found, as the result of subsequent changes effected in these strata, trans- formed into gneiss, hornblende-schist or mica-shist, by what is vaguely designated as met.imorphism. The recent history of geology abounds in striking illustrations of the fact that in a great number of cases these views have been based on misconceptions in stratigraphy, and without entering into the discussion of the question, it may be said that, in the writer's opinion, careful stratigraphieal study will, in all cases, suffice to show the error, both of the plutonic and the metamor- phic hypotheses of the origin of crystalline rocks. The former is supported chiefly by the lithological resemblances between certain stratified and unstratified rocks, and by the appearances of stratification occasionally found in these ; while the latter is HUNT — PRE-OAMBRIAN ROCKS. 'J I'aus- It is I the |scs, 10 r- icr 3en Ices is sustained by the analogies oiFered in cases of local hydn thermal action on sediments, and by the resemblances which reuomposed materials frequently offer to their parent crystalline rocks. It is here maintained that the great formations of stratiform crys- talline feldspathic, hornblendic and micaceous rocks, which, in various parts of the world, have been alternately described as plutonic masses, and as metamorphosed paleozoic, mcsozoic or cenozoic strata are, in all cases, neptuuean rocks, preCambrian or pre-Silurian in age, and that we know of no uncrystallino sediments which are their stratigraphical equivalents. We have then before us two schools, the one maintaining the secondary origin of a great, and, by them, uudefiued portion of the crystalline stratiform rocks, while assigning to certain older (pre-Cambrian) crystalline rocks (of W'hich they admit the ex- istence), either a neptunean or u plutonic origin. The other, or ] lutonist school, while asserting the plutonic derivation of the greater part of the crystalline formations, accepts, to some extent also, the notion of secondary and neptunean mctamorphie schists. It is believed that the above concise statements cover the ground held by the hitherto prevailing neptunean and plutonist schools, neither of which, it is maintained, expresses correctly the present state of our knowledge. In opposition to both of these are the views taught for the last twenty years by the writer, and now ac- cepted by many geologists, which may be thus defined : — l.s^ All gneisses, petrosilexes, hornblendic and micaceous schists,* olivines, .serpentines, and in short, all silicated crystalline stratified rocks, are of neptunean origin, and are not primarily due to metamorphosis or to metasomatosis either of ordinary aqueous sediments or ot volcanic materials. 2d. 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 * It is a question how far the origin of siuli cry stal lint; aluminous silicates as muscovite, margarodite, damourite, pyropliyllite, l^yauite, librolite and andalusito is to Ito sought in a process of diagciicsis in ordinary aqueous sediments iiolding tlie ruins of more or less com. pletely decayed feldspars. Other aluminous rock-forming silicates, such as chlorites and raagnesiau micas, are however connected, through aluminiferous amphiholes, with the non-aluminous magne- sian silicates, and to all of these various magnesiau minerals a very different origin must be ascribed. 4 HUNT — PRE-CAMBRIAN ROCKS. affinitius) have not been reproduced to any great extent since the begintiinjr of paleozoic time. 3J. The eruptive rocks, or ut least a large part of them, are softened and displaced portions of these ancient ncptunean rocks, of which they retnin many oi" the mineralogical and lithological characters. II. The History of pre-Cambrian Rocks in America. Coming now to the history of our knowledge of American crystalline rocks, we find that the lithological characters of the Primary gneissic formation of northern New York were known to iMaclure in 1817, and were clearly defined in 1832 by Eaton, who, under the name of the Macomb Mountains, described what have since been called the Adirondacks, and moreover distin- guished them from the Primary rocks of New England. Emmons, in 1842, added much to our lithological knowledge of the cry- stalline rocks of northern New York, but regarded the gneisses, with their associated limestones, serpentines and iron-ores as all of plutonic origin. Nut tall, who had previously studied the similar rocks in the Highlands of southern New York and New Jersey, had however maintained, as eurly as 1822, that these had resulted from an alteration ot the adjacent paleozoic grayvvackes and lime- stones, into which he supposed them to graduate. This view was, at the time, opposed by Vanuxem and Keating, but was again set forth in 1843, by Mather, who while admitting the ex- istence of an older or Primary series of crystalline rocks, con. ceived a great part of these rocks in southcsrn New York to be altered paleozoic, and distinguished them as Metamorphic rocks. To this latter class he referred all the crystalline stratified rocks of New PJiigland, and ended by doubting whether a great part of what he had described as Primary was not to be included in his Metamorphic class. The subsequent labors of Kitchell and of Cooke have however clearly established the views of Vanuxem and Keating as to the Primaiy age alike of the gneisses and the crystalline limestones of the Highlands. The similar gneissic series in Canada, which was known to Bigsby and to Eaton as an extension of that of northern New York, was noticed by Murray in 1843, and by Logan in 1847, as pre-paleozoic, though apparently of sedimentary origin, and hence, according to them, entitled to be called Metamorphic rather tlian Primary. It was described by Logan in 1847, as HUNT — PRE-CAMBRIAN ROCKS. consisting of a lower group of hornblendic gneisses witliout lime- stouea, and an upper group of similar gneisses, distinguished by interstratified crystalline limestones. These rocks were found by Logan and by Murray to be over- laid, both on Lake Superior and in the valley of the upper Ottawa, by a series consisting of chloritic and epidotic schists, with bedded greenstones, and with couglomeratef holding pebbles derived from the ancient gneiss below. The same overlying series had, as early as 1821. been described by Bigi^by on Lake Superior, and by him distinguished from the Primary and classed with Transition rocks. Labrau.fiitic and hypersthenic rocks like those previously de scribed by Emmons in the Primary region of northern New York, were, in 1853 and 185-4, discovered and carefully studied in the Laurentide hills to the north of Montreal, when they were described as being gneissoid in structure, and as interstratified with true gneisses and with crystalline limestones. Li 1854, the writer, in concert with Logan, proposed for the ancient crystal- line rocks of the Laurentide Mountains, including the lower and upper gneissic groups already mentioned, and the succeeding labradoritic rocks (but excluding the chloritic and greenstone series), the name of Lauren tian. In an essay by the writer, in 1855, the oldest gneisses of Scotland and Scandinavia were, on lithological and on stratigraphical grounds, referred to the Laur- entian series, and at the same time the name of Huronian was proposed for the chloritic and greenstone series, which had been shown to overlie unconformably the Laurentian in Canada. Previous to this, in 1851, Foster and Whitney had described the Laurentian and Huronian rocks of Lake Superior as consti- tuting one Azoic system of Metamorphic rocks, with granites, porphyries and iron-ores of igneous origin ; and in 1857, Whitney attacked the two-fold division adopted by the Canadian geological survey, maintaining that the stratified crystalline rocks of the region belong to a single series, with a granitic nucleus. The observations of Kimball in 1865, and the later studies of Credner, of Brooks and Pumpelly, and of Irving, have, however, all con- firmed the views of the Canadian survey as to the relations of the Laurentian and Huronian in this region. The primary sige of the Highlands of southern New York, aad their extension in what is called the South Mountain, as far as the Schuylkill, was now unquestioned, but the crystalline rocks* 6 HUNT — PRE-CAMBRIAN ROCKS. to the east of this ranfic, while re^;ard(!d by Ealon and by Em- mons, as also forniiuj'' u part of the Primary, were, by Mather, as we have already seen, supposed to be altered paleozoic strata. These rocks in New Enj:;lund, with the exception of the quartzites and limestones of the Taconic range, were by him assigned to a horizon above the Trenton limestone of the New York system, and portion.s of them were conjectured by other geologists, who adopted and extended the views of Mather, to be of Devonian age. The characteristic crystallin( chists of New England and soutliciistern New York, passing beneath the Mesozoic of New Jersey, reappear in southeastern Pennsylvania, where they were studied and finally described by H. D. Rogers in 1858. Ac- cording to him, these crystalline ^chists, while resting uncon- formably upon iin ancient (Hypozoic) gneissic system, were themselves more ancient than the Scolithus-sandstone, which he regarded as the equivalent of the Potsdam. While he supposed these newer crystalline schists, called by him Azoic, to be con- nected stratigraphically with the base of the Paleozoic series, he nevertheless assigned them to a position below the base of the New York system ; thus recognizing in Pennsylvania, beneath this horizon, two unconformable groups of crystalline rocks, cor- responding stratigraphically as well as lithologically, with the Laurentian and the Huronian of the Lake Superior region. The existence among these newer crystalline schists of Penn- sylvania, of a series distinct from the Huronian, and i-epresenting the White Mountain or Montalban rocks (the Philadelphia and Manhattan gneissic group), had not been then recognized. Rogers at this time taught the igneous origin of the magnetic iron ores, the quartz-veins, the serpentines and their associated greenstones in this region. The belief entertained by Rogers of an intimate connection between his upper or Azoic series and the Paleozoic, had its origin, apparently, in the fact of the exist- ence in this region of still another and a newer crystalline series, the ]iO\ver Taconic of Emmons, or the Itacolumite group of Lieber, which I have designated Taconian, and propose to con- sider in detail in a future paper. In it are included the iron- ores of Reading, Cornwall and Dillsburg, in Pennsylvania. The views of H. D. Rogers with regard to the crystalline schists of the Atlantic belt were thus, in effect, if not in terms, a return to those held by Eaton and by Emmons, but were in direct opposition to that maintained by Mather, which had been adopted HUNT — PRE-CAMBRIAN ROCKH. by Logan, and b}' the present writer. The belt of micaceous, chloritic. talcosc and epidotic schists, with greenstones and ser- pentines, the extension of a part of the Azoic of Rogers, which, through western New England, is traced into Canada, (where it has been known as the Green Mountain range), was previous to 18(52 called by the geological survey of Canada, Altered Hudson- River group. It was Hubsequently referred to the Upper Taconic of Emmons, to which Logan, at that date, gave the name of the Quebec group, assigning it, as had long before bo3, under the name Coldbrook gi'oup, which included a lower and an upper division. In a joint report of Matthews and Bailey in 18G5, these rocks were declared to be overlaid unconformably by the slates in which Hartt had made known a Lower Cambrian (Meuevian) fauna, and were compared with the Huronian of Canada. The lower division of the Coldbrook was then described as including a large amount of pink feldspathic quartzite and of bluish and reddish porphyriti^ slates. In the same report was described, under the name of the Bloomsbury group, a series lithologically s HUNT — PRE CAMBRIAN ROCKS. fiimilnr to the Coldbrook, but apparently resting on the Menevian, and overlaid by fossiliferouo Upper Devonian beds, into which it was supposed to graduate. The Bloomsbury group was there- fore regarded as altered Upper Devonian, and its similarity to the pio-Canibrian Coldbrook was explained by supposing both groups to consist in large part of volcanic rocks. In 1869 and 1870, however, the writer, in company with the gentlemen just named, devoted many weeks to a careful study of these rocks in southern New Brunswick, when it was made apparent that the Bloom^ibury group was but a repetition of the Coldbrook on the opposite side of a closely folded synclinal hold- ing Menevian sediments. These two areas of pre-Cambrian rocks were accordingly described by Messrs. Matthews and Bailey in their report to the geological survey of Canada in 1871, as Huronian, in which were also included the similar crystalline rocks belonging to two other areas, which had been previously described by tlie same observers under the names of the Kingston and Coastal groups, and by them regarded aS respectively altered Silurian and Devonian. After studying the Huronian rocks in southern New Bruns- wick, and their continuation along the eastern coast of New England, especially in Ma.ssachusett>< (where, also, they are over- laid by Menevian sediments), tlie writer in 1870, announced his conclusion that the crystalline schists of these regions are lithol- ogicaliy and stratigraphically eears in a paral- lel range farther east, which extends southward into New Hamp- shire. In his tabular view of the geognostical groups in this State, Hitchcock assigns to these rocks a thickness of over 12,000 feet, with the name of Upper Huronian ; while he designates as Lower Huronian the petrosilex series of esistern Massachusetts, already noticed, where these rocks are of groat, though undeter- mined, thickness. The similar petrosilex or hiilleflinta rocks in Wisconsin, where they have lately been described by Irving as Huronian, have according to this observer, a thickness, in a single section, of 3,200 feet. They here sometimes become 10 HUNT — PRE-CAMBRIAN ROCRS. ,«chist()?o, ;ind are interbedded with unctuous schists, and rest in ;ippavent conformity upon a great mass of quartzite. The general high inclination both of this series and of the typical Huronian, renders the determination of their thickness difficult. The maximum thickness of the Huronian (excluding the petro- silex scries) to the south of Lake Superior, may. according to Major Brooks, exceed 12,000 feet, while the estimates of Credner and Murray, respectively, for this region, and for the north shore of Lake Huron, are 20,000 and 18,000 feet. As regards the Laurentian, there exists a certain confusion of nomenclature which requires explanation. As originally descri- bed, it includes, as already said, a basal granitoid gneiss, with- out limestones, which the writer has elsewhere designated the Ottawa gneiss, and of which the thickness is necessarily un- certain. Succeeding this is the Grenville series of Logan, having for its base a great mass of crystalline limestone, and consisting in addition to this of gneisses, generally hornblendic, and quart- zites, interstratificd with similar limestones. To thi.s series, as displayed north of the Ottawa, Logan assigned an aggregate thickness of over 17.000 feet, thouuli the later measurements of Vennor, in the region south of the Ottawa, give to it a much greater volume. The geographical distribution of this limestone- bearing Grenville series gives probability to the suggestion of Vennor that it rests uncomformably upon the bisal Ottawa gneiss. Thes two divisions constitute what was designati d by Logan, in his Geological Atlas, in IHtif). the Lower Laurentian, — the name of Upper Laurentian or Labradorian being then, for the first time given by him to a series supposed to overlie uncon- formabl}' the foru)er. of which it had hitherto been regarded as constituting a part. This third division has already been referred to as characterized by the predominance of great bodies of gneissoid or granitoid rocks, composed ciiiefly of labradorite or related anorthic feldspars, and apparently identical with the norites of Scandinavia. With these basic rocks are inter- stratified crystalline limestones, quartzites and gneisses, all of which resemble lho.se of the Grenville series. This upper group, for which the writer in 1871 proposed the name of Norian, was supposed by Logan to be not less than 10,000 feet thick. For farther details of the history of these various groups of pre Cambrian rocks, and their distribution in North America, HUNT — PRE-CAMBRIAN ROCKS. 11 the reader is referred to ;i volume published iu 1878 by the Sccoud Gooloj^iciil Survey ol' Pennsylv.inia, beinu: Part I of the writer's report on Azoic Eocks, intended as an historical intro- duction to the subject. III. — The History of Pre-Cambrian Rocks in Grkat Britain. In an addre^is before this Associntion in 1871, in which the writer maintained the Huronian age of a portion of the crys- talline schists of New En<;lrind and Quebec, he further expressed the opinion, based in part upon his examinations at Holyhead in 1867. and in part upon the study of collections in London, that certain crystalline schists in North Wales would be found to belong to the Huronian series. The rocks in question were by Sedgwick, in 18IJ8, separated from the base of the Cambrian, as belonging to an older series, but were subsequently, by Dela- beche, Murchison and Ramsay, described and mapped as altered Cambrian strata, with associated intrusive syenites' and feldspar- porphyries. In South Wales, at St. David's in Pembrokeshire, is another area of crystalline rocks, which the geological .survey ol" Great Britain had mapped as intrusive syenite, granite and felstone (petrosilex-porphyry) having Cambrian strata converted into crystalline .«chists on one side, and unaltered fossiliferous Cam- brian beds on the other. So long ago as 1864, Messrs. Hicks and Salt:i ; Hughes & Bonney, Feb. ISTO,. p. 137; Hicks & Davies, May 1879. i>. 285; Hicks & Bonney, ibid, p. 295 ; Bonney, ibid, p. 309 ; Bouncy iV Houghton, ibid, p. 821 ; Hughes,, Nov. 1879, p 682 ; Maw, Aug. 1878, p. 7G4; also Hicks, rocks of Ross- shire, Nov. 1878, p. 811. Tawuey, Older Hooks of St. Davids: Proc. Bristol Naturalists' Society, vol. II, part 2, p. 110. On these rocks in Shropshire, in the same .Journal, Alli)ort, Aug. 1877, p. 449; Callaway, Nov. 1877, p. 653, and Aug. I87», p. T54; Callaway & Bonney, Nov. 1879, p. 643. HUNT — VRE-CAMBRIAN ROCKS. 17 On these lookH in Charnwood Forest, iu the same Journal, Hill k Bonney, Nov. 1887, p. 753, and May, 1878, p. 199. See farther. Hunt, Chemicul and Geological Essays, pp. 34, 2<)9, 270, 272, 278, 383; also his Azoic Rocks, part I (Second Geol. Survey of Penn., 1878), pp. 187, 188. For the rocks of the Ardennes see Memoir sur les Roches dites Plutoniques, etc. (4to, pp. 264), by de la Vallee Poussin and Renard, from Memoi res de r Acad. Royale de la Belgique for 1876; Memoire sur la Comp. Mineralogique du Coticule, by Renard, from the same for 1877; and The Minenilogical and Microscopical Characters of the Belgian Whetstones, by Renard, Monthly Microscopical Journal for 1877, Vol. xvii. p. 269. Also Gosselet and Malaise, Terrain Silurian des Ardennes, Bull. Acad. Roy. de laBelgique (2) No. 7, 1868 ; Dewalque, Terrain Cambrien des Ardennes, Ann. Soc. Geol. de la Belgique, torn. I, p. 63 ; and larther, Hunt, Chem. and Geol. Essays, p. 270.] Appendix. Since the above paper wns read the author has received (No- vember, 1879) a private commUDication from Prof. L. W. Bailey, giving his latest results as to the pre-Cambriau rocks of southera New Brunswick, which confirm what has already been said about that region. Bailey separates the Huronian into a lower division, for which he reserves the name of Coldbrook, consisting chiefly of petrosilex rocks, and an upper division, the typical Huronian, called by him the Coastal group. He adds that there is between the two a marked physical break, which is indicated by a stratigraphical discordance, and by the presence in the lower part of the Coastal group of coarse conglomerates made up from the ruins of the Coldbrook or underlying division. This correspond to the break between the similar Arvonian and Hu- ronian in South Wales. At the meetiiij, of the British Association for the Advance- ment of Science at Sheffield in August, 1879, Dr. Hicks read a paper on the Classification of the British Pre-Cambrian Rocks, which is published iu the Geological Magazine for Octo- ber, 1879. He concludes that the Pebidian is "a group of enormous thickness, which is largely distributed over Great Britain, where it has a prevailing strike of N.N.E. and S.S.W., or from this to N.E. and S.W." In addition to the localities which we have already mentioned in Great Britain, he notes its occurreuce in Shropshire and in Charnwood Forest, and also in the northwest of Scotland, where, as elsewhere, it enters largely 18 HUNT — PRE-OAMimiAN ROCKS. into tlie Lower Cauib»-iaii coii^loincrates. The group is con- cisely described by him as consistiuf!; " for the most part of'chlo- ritic, talcose, feldspathic nud micaceous schi^^tosc rocks, alternating with shity and massive greenstones, dolomitic limestones, serpen- tines, lava-flows, porcellanites, breccias and conglomerates. It is also traversed frequently by dykes of granite, dolerite, etc." The conglomeratcvs .it the base of the Huroniau in Wales are largely make up of the masses derived from the Arvonian, with whioh " it is undoubt(Klly, ;;t most of the points examined, uncon- formable." This x\.rvonian series, Hieks regards as identical with the great Hillleflinta group of the Swedish geologists and with the Petro.«ilex scries which the writer lias made known in America. In addition to the localities already mentioned of it in the British Isles, Hicks notes its occurrence in the Harlech Mountains and the Orkneys, and probably also in the Western Islands, and in the Grampians of Scotland. Its strike in the regions examined by him is generally about N. and S. As regards the gnei.ssic Dimetiau group, the strike of which is N.W. and S.E., or from this to N. and S., Hicks add.>. to the localities in Wales, already noticed, its occurrence in the Mal- vern chain, especially in the Worcester Beacon, and cites Dr. Callaway as authority for its existence in Shropshire. Hicks further notes its presence in several points in the northwest Highlands of Scotland. From this series of light colored gneisses, often very quartzose, with limestone bands, he separates, as we have seen, under the name of Lewisian, proposed by Murchison for the ancient gneisses of Lewis and otlu-rs of the Hebrides Isles, these, and similar reddish and dark-colored hornblendic gneisses which are found in parts of the Malvern chain, in the northwest of Ireland, and pos.sibly also in iVnglesey. This .series, according to Hicks, is unconformably overlaid by the Dimetian, brecciated beds in which ht>ld fragments of the older Lewi.>