Piil A^ ^%. v^, IMAGE EVALUATION TEST TARGET (MT-3) V A ^. hly metal- liferous strata show the existence of shallow water, lagoons and springs over a great area and for a long period betweon the forma- tion of the upper and lower shales. We may suppose that while the Potsdam sandstone was being depasited along the shoves of the great paleozoic ocean, the lower black shales were accumula- ting in the deeper waters, after which an elevation took place, anti the magnesian strata were deposited, followed by a subsidence during the period of the u-.per shales and Sillery sandstones. Associated with the magnesian strata at Poiit Levis and in several other localities in the same horizon of the Quebec group, an extensive fauna is found, of which 137 «pecies are now known, embracing more than torty new species of graptolites, which have been described by Mr. James Hall in the report of the Geological Survey of Canada for 1857, and thirty-six ipocies of trilobites described by Mr. B'Uings in the Can^idian Naturalist for August 16G0. These species are as yet distinct from anything found in the Potsdam below or the Birdseye and Black River above; * See Sir Williom Logan's letter tc Mr. Barrande, Uauadlan Naturalist for Jan. 1861, and Am. Journal, of Science (2) xxxi. 216. < i on American Geology, n S it 1 although the trilobites recall by their aspect those found by Oweu in the Lower Sandstone of the Mi-^sissippi. Seven species alone out of this fauna have been identihed with those known in other formations, and of these one is Chazy, while six belong to the Calciferous, to which latter horizon Mr. Billings considers the Quebec group to belong. The Chazy has not yet been identified in this region, unless indeed it be represented in some of the upper portions of the Quebec group. The Calciferous sand-rock is want- ing along the north side of the St. Lawrence valley from near Lake St. Peter to the Mingan Islands, but at Lorette beliind Que- bec, at the foot of the Laurentides, the ]3irdseye limestone is found reposing conformably upon the Potsdam sandstone. It is not easy to find the exact horizon of the Potsdam sandstone among the black shales which underlie the Quebec group. The Scolithus of Rogers' Primal sandstone, and of the summit of Safford's 3rd or Chilhowee formation is identical with that found in the quartz rock at the western base of the Green Mts, and fig- ured by Mr. Hall in the 1st volume of the Paleontology. It is however distinct from what has been called /S^coZiV/ius in the Pots- dam of Canada. The value of this fossil as a means of identifi- cation is diminished by the fact that similar marks are found ia sandstones of very diff"erent ages. Thus a Scolithus very like that of the St. Lawrence valley occurs in the sandstone of Lake Superior and in the Medina sandstone, while in Western Scot- land, according to Mr. Salter, the two quartzite formations above and below the Lower Silurian limestones of Chazy age are alike characterized by these tubular markings, which are regarded by him as produced by annelids or sea-worms. We find however in shales which underlie the Quebec group at Georgia in Vermont, trilobites which were described by Mr. Hall in 1859 as belong- ing to the genus Olenus, a recognized primordial type ; he has since erected them into a new genus. Again at Braintree in Eastern Massachusetts occur the well known Paradoxides in an aigilla- ceous slate. These latter fossils Mr. Hall suggests probably belong to the same horizon as certain slaty beds in the Potsdam sand- stone, or perhaps even at the base of this formation. (Introduction, page 9.) In this connection we must recall the similar shales of Newfoundland,in which Salter has recognized trilobites of the same genus. These shales containing Paraloxides, like those undeilying the Quebec group, thus appear to belong to the so-called Primor- dial zone, and are to Le regarded ps the equivalents of the Potsdam 12 Mr. T. Sterry Hunt sandstone, which both on Lake Champlain and in the Mississippi valley is characterized by primordial types. The intermingling of Potsdam and Calciferous forms to which we have already alluded, seems however to show that it will be difficult to draw any well defined zoological horizon between the different portions of these lower rocks, which at the same time offer as yet no evidences of any fauna lower than that of the Potsdam. So that we regard the whole Quebec group with its underlying Primordial shales as the greatly developed representative of the Potsdam and Calciferous (with perhaps the Chazy), and the true base of the Silurian system. The Quebec group with its underlying shales is no other than the laconic system of Emmons. Distinct in their lithological characters from the Potsdam and Calciferous formations as devel- oped on Lake Champlain, Mr. Emmons was led to regard these strata as belonging to a lower or sub-Silurian group. We have how- ever shown that the paheontological evidence atlbrded by this formation gives no support to sucli a view. To Mr. Emmons is however undoubtedly due the merit of having for a long time maintained that the Taconic hills are composed of strata inferior to the Trenton limestones, brought up into their present position by a great dislocation, with an upthrow on the eastern side. We would not object to the term Taconic if used as indicating a sub- division of the Lower Silurian series, but as the name of a distinct and sub-Silurian system it can no longer be maintained. The Quebec group evidently increases in thickness as we proceed to- wards the south, and the calcareous parts of the formation are more developed. In 1859, 1 visited in company with Mr. A. D. Ea- ger the marble quarries of Rutland and Dorset, in Vermont. The latter occur in a remarkable syncliual mountain of nearly horizontal strata of marble and dolomite, capped by shales, and attaining a height of 270;> feet above the railway station at its base. I then identified these marbles with the limestones of the Quebec group, considering them to be beds of chemically precipitated carbonate of lime or travertine, and not limestones of organic origin. The existence of great dislocations in the Appalachian chain is amply illustrated in the sections of Prof. Rogers, and in those given by Saftbrd in Eastern Tennesse, where by the aid of fossils it becomes comparatively easy to trace them. See the Map accorn- patiyintj' his Geological Recoiinaissance of Tennessee, 1855; vvhere the magnesian limestcnes of formation IV, are shown to be not ^ % / -V n/ on American Geology, 13 V f / ^ il/ only brought up on the east against the Upper Silurian and De- vonian, but even to overlap the black shales at the base of the Car- boniferous system. It is remarkable to find that as early as 1822, the idea of a great dislocation of this nature in Eastern New York was maintained by Mr. D. H. Barnes in his description of Canaan Mountain. [Am. Journal of Science, (1) v. pp. 15-18.] To the southeast of this great fault in Canada we have as yet no evidence of Lower Silurian strata higher than those of the Quebec group. At the eastern base of the Green Mts. we find limestones of upper Silurian and Devonian age reposing uncon- formably upon the altered strata of the Quebec group, themselves also having undergone more or less alteration. Immediately suc- ceeding are the chiastolite and mica slates of Lake St. Francis, which as we have long since stated are probably also of Upper Silurian asre. The White Mountains as we suggested in 1849, (Am.Jour.Sci. (2) ix. 19) are probably, in part at least, of Devonian age, and are the representatives of the 7000 feet of Devonian sandstone observed by Sir William Logan in Gaspd. Mr. J. P. Lesley has more re- cently, after an examination of the White Mts. shown that they possess a synclinal structure, and has adduced many reasons fo'r regarding them as of Devonian age. (Amer. Mining Journal, January 1861, p. 99. It will be seen from what has been previously said that we look upon the 1st and 2nd divisions described by Mr. Safford in Eastern Tennessee, as corresponding to the hypozoic series of Rogers and to the Green Mountain gneissic formation, which instead of being beneath the Silurian series, is really a portion of the Quebec group more or less metamorphosed, so that we re- cognize nothing in New England or south-eastern Canada lower than ^the Silurian system, nor do we at present see any evidence of older strata, such as Laarentian or Iluronian, in any part of the Appalachian chain. The general conclusions which we have previously expressed with regard to the lithological, chemical and mineral relations of the Green Mts. rocks remain unchangetl. [Am. Journal of Science (2) ix. 12.] The remarkable parallelism between the rocks of Western Scot- land and Canada has already been shown in the existence of the Laurentian, and Cambrian (Huronian) systems, overiaid byquartz- ites containing Smlithm, to which succeed limestones containing a numerous fauna, identified by Mr. Salter with that of the Chazy 14 Mr, T. Sterry Hunt limestone. These strata, with an eastward dip, are covered by other quart zites nnd limestones, to which succeeds the great gneissoid formation of the western Highlands, consisting of feldspathic, chlo- ritic, micaceous, and ^alcose schists resembling closely the gneissoid rocks of ihe Green Mts. and including the chromiferous ophiolites of Perthshire, Banff and the Shetland Isles. This gneissoid series was by Prof. Nicol suggeste*' to be the older or Laurentian gneiss brought up by a dislocation on the east of the Silurian limestones, but Sir Roderick Murchlson, with Messrs. Ramsay and Harkness, has shown not only from the dif- ferences in lithological character, but from actual sections, that the eastern gneissoid series is made up of altered strata newer than the Silurian limestones.* Thus in geological structure aud age, not less than in litliolo2;ical and mineralogical characters, the rocks of the western Highlands are the counterparts of the Lau- rentian and Silurian gneiss formations, as seen in the Laurentides and Adirondacks, and in the Green Mts. The same parallelism may be extended to Scandinavia, (where Kjerulf and Forbes have shown much of the crystalline gneiss to be of Silurian age,) mark- ing as it would seem the outer edge of a vast Silurian basin, which may be followed in the other direction across the Atlantic to the Gulf of Mexico. We also remark in Great Britain as in America, that whereas the northern outcrop of the palaeozoic basin offers at its base only a series of quartzose sandstones reposing upon the Laurentian system and characterized by fucoids and Scolithus, we find further south in England an immense development of shales, sandstones and conglomerates, which form the base of the Silurian system and correspond to the Primordial zone and the Quebec group. We have said that upon Lake Huron and Superior the sand- stones of the upper copper-bearing rocks are the equivalents of the Quebec group. The clear exposition of the question by Mr, J. D. Whitney in the Am. Mining Jour, for 1860 (p. 435) left little more to be said, but the sections made last year by Mr. Alex. Murray of the Canada Geological Survey place the matter beyond all doubt. On Campment d'Ours, a small island near St. Joseph's, the sand- stones of Sault St. Mary are seen reposing horizontally on the upturned edges of the Huronian rocks, and overlaid by limestones which contain in abundance the fossils of the Black River and * Murchison, Quar. Jour. Geol. Society, Vol. xv. 353 and xvi. 215. \ on American Geology, Jg Birdseye divisions. The only fossil as yet found in those sand- stones is a single Lin^/ula from near Sault St. Mary, which may be either of Potsdam or Chazy age. The sandstones in question form the upper member of a series of strata which on Lake Supe- rior attain a thickness of several thousand feet, and passing down- wards we find a succession of limestones, marls and argillaceous sandstones, interstratified with greenstone and amvgdaloid, and fol- lowed by about 2000 feet of bluish slates and'sandstones, with cherty beds containing grains of anthracite, the whole underlaid by conglomerates, and reposing unconformably upon rocks of the H«- ronian system. The presence of sucli slates is the more significant from the occurrence already mentioned of fragments of green and black slates in the coarse grained sandstones near the base of the Potsdam, at Hemmingford mountain, showing the existence of ar- gillaceous shales before the deposition of the quartzites of the Pots- dam ; these are perhaps more recent than the lowest shales of the Primordial zone, to which however, palaeontologically they appear to belong. This Quebec group is of considerable economic interest inas- much as it is the great metalliferous formation of North America. To it belongs the gold which is found along the Appalachian chain from Canada to Georgia, together with lead, zinc, copper, silver, cobalt, nickel, chrome and titanium. I have long since called attention to the constant association of the latter metals, particu- larly chrome and nickel, with the ophiolites and other magnesian rocks of this series, while they are wanting in similar" rocks of Laurentian age. Am. Jour, of Science (2) xxvi. 237. The immense deposits of copper ores in Eastern Tennessee, and the similar ones in Lower Canada, both of which are for the most part in beds subordinate to the stratification, belong to this group. The lead, copper, zinc, cobalt and nickel of Missouri, and the copper of Lake Superior, also occur in rocks of the same age, which ap- pears to have been pre-eminently the metalliferous period. The metals of the Quebec giojp seem to have been originally brought to the surface in watery solution, fr.un which weco^nceire them to have been s.^parated by the reducing agency of organic matter in the form ..i alphurets, or in the native stat«, and mingled ■with the contemporaneous sediments, where they occur in beds in disseminated grains iormmg faklbands, or as at Acton, are the cementing material of conglomerates. During the subsequent metamorphism of the strata these metallic matters being taken 16 Mr, T. Sterry Hunt into solution by alkaline carbonates or sulphurets, have been redeposited in fissures in the metalliferous strata, forming veins, or ascending to higher beds, have given rise to metalliferous veins in strata not themselves metalliferous. Such we conceive to be in a few words the theory of metallic deposits ; they belong to a period when the primal sediments were yet impregnated with metallic compounds which were soluble in the permeating waters. The metals of the sedimentary rocks are now however for the greater part in the form of insoluble sulphurets, so that we have only traces of them in a few mineral springs, which serve to show the agencies once at work in the sediments and waters of the earth's crust. The present occurrence of these metals in waters which are alkaline from the presence of carbonate of soda, is as we have elsewhere pointed out, of great significance when taken in connection with the metalliferous character of certain dolomites, which as we have shown, probably owe their origin to the action of similar alkaline springs upon basins of sea water. The intervention of intense heat, sublimation and similar hy- potheses to explain the origin of metallic ores, we conceive to be uncalled for. The solvent powers of solutions of alkaline carbonates, chlorids and sulphurets at elevated temperatures, taken in connection with the notions above enunciated, and with De Senarmont's and Daubr6e's beautiful experiments on the crys- tallization of certain mineral species in the moist way, will sufiice to form the ba.sis of a satisfactory theory of metallic deposits* The sediments of the carboniferous period, like those of earlier formations, exhibit towards the east a great amount of coarse sedi- ments, evidently derived from a wasting continent, and are nearly destitute of calcareous beds. In Nova Scotia Sir William Logan found by careful measurement, 14,000 feet of carboniferous strata ; and Professor Rogers gives their thickness in Pennsylvania as 8000 feet, including at the base 1400 feet of a conglomerate, which disap- pears before reaching the Mississippi. In Missouri Prof. Swallow finds but 640 feet of carboniferous strata, and in Iowa their thick- ness is still less, the sediments composing them being at the same time of finer materials. In fact, as Mr. Hall remarks, throughout the whole palaeozoic period we observe a greater accumulation and a coarser character of sediments along the lino of the Appa- lachian chain, with a gradual thinning westward, and a deposition of finer and farther transported matter in that direction. To the / ft • Quar. Jour. Geol. Soc. vol. xv. 580. on American Geology, 17 west, as this shore^erived material diminishes in volume, the amount of calcareous matter rapidly augments. Mr. Hall concludes therefore that the coal-measure sediments were driven westward luto an ocean, where there already existed a marine fauna. At length, the marine limestones predominating, the coal measures ^ come to be of little importance, and we have a great limestone formation of marine origin,which in the Rocky Mountains and New Mexico occupies the horizon of the coal, and itself unaltered, rests on crystalline strata like those of the Appalachian range. In truth, Mr. Hall observes, the carboniferous limestone is one of the most extensive marine formations of the continent, and is charac- terized over a much greater area by its marine fauna than by its terrestrial vegetation. "The accumulations of the coal period were the last that gave form and contour to the eastern side of our continent, from the Gulf of St. Lawrence to the Gulf of Mexico ; and as we have shown that the great sedimentary deposits of successive periods have followed essentially the same course, parallel to the mountain ranges, we naturally inquire : What influence this accumulation has had upon the topography of our country, and whether the present line of mountain elevation from north-east to south-west 18 m any way connected with the original accumulation of sediments?" ITalVs Introduction, p. 66. The total thickness of the paleozoic strata along the Appala- chian chain is about 40,000 feet, while the same formations in the Mississippi valley, including the carboniferous limestone, which is wanting in the east, have according to Mr. Hall, a thictkness of scarcely 4000 feet.* In many places in this valley we find the Silurian formations exposed, exhibiting hills of 1000 feet, made up of horizontal strata, with the Potsdam sandstone for their base, and capped by the Niagara limestone, while the same strata* in the Appalachians would give from ten to sixteen times that * In Michigan, according to the late report of Prof. Winchell, the total observed thickness of the strata from the top of the Sault St. Mary sandstones to the top of the carboniferous series is little over 1790 feet divided as follows :-Trenton and Hudson River groups, 50 feet, Upper Silunan 185, Devonian 782, Carboniferous 700; of this last the true coal measures constitute 123 feet, including from 3 to 10 feet of workable bituminous and cannel coals, while near the base of the carboniferous aeries are lound 169 feet of gypsiferoua marls, which yield strone brine springs. IS Mr. T. Sterry Hunt thickness. Still, as Mr. Hall remarks, we have there no mountains of corresponding altitude, that is to say, none whose height like those of the Mississippi valley, equals the actual vertical thick- ness of the strata comprising them. In the west there has been little or no disturbance, and the highest elevations mark essentially the aggregate thickness of the strata comprising them. In the disturbed regions of the cast on the contrary, though we can prove that certain formations of known thickness are included in the mountains, the height of these is never equal to the aggre- gate amount of the formations. " We thus find that in a country not mountainous, the elevations correspond to the thickness of the strata, while in a mountainous country, where the strata are im- mensely thicker, the mountain heights bear no comparative pro- portion to the thickness of the strata." '• While the horizontal strata give their whole elevation to the highest parts of the plain, we find the same beds folded and contorted in the mountain region, and giving to the mountain elevations not one-sixth of their actual measurement." Both in the east and west, the valleys exhibit the lower strata of the palajozoic series, and it is evident that had the eastern region been elevated without folding of the strata, so as to make the base of the series correspond nearly with the sea level, as in the Mississippi valley, the mountains exposed between these valleys, and including the whole pal860zoic series, would have a height of 40,000 feet; so that the mountains evidently correspond to depressions of the surface, which have carried down the bottom rocks below the level at which we meet them in the valleys. In other words, the synclinal structure of these moun- tains depends upon an actual subsidence of the strata along certain lines. " We have been taught to believe that mountains are pro- duced by upheaval, folding and plication of the strata, and that from some unexplained cause these lines of elevation extend along certain directions, gradually dying out on either side, and subsiding at the extremities. We have, however, here shown that the line of the Appalachian chain is the line of the greatest accu- mulation of sediments, and that this great mountain barrier is due to original deposition of materials, and not to any subsequent forces breaking up or disturbing the strata of which it is composed." We have given Mr. Hall's reasonings on this subject, for the most part in his own words, and with some detail, for we \ '► / ,• on American Geology, V 19 conceive that the views which he is here urging are of the highest importance to a correct understanding of the theory of mountains.- In the Canadian Naturalist for Dec. 1859, p. 425, and in the Am. Jour. ScL (2) xxx, 137 will bo found an'allusion to the rival theories of upheaval and accumulation as ap- plied to volcanic mountains, the discussion between which we conceive to be settled in favour of the latter theory by the reasonings and observations of Constant-Prevost, Scrope and Lyell. A similar view applied to mountain chains like those of the Alps, Pyrennees and Alleghanies, which are made up of aqueous sediments, has been imposed upon the world by the autho- rity of Humboldt, Von Buch and Elie de Seaumont, with scarcely a protest. Buffon, it is true, when he explained the formation of continents by the slow accumulation of detritus beneath the ocean, conceived that the irregular action of the water would give rise to great banks or ridges of sediments, which when raised above the waves must assume the form of mountains; later, in 1832, we find De Montlosier protesting against the elevation hypothesis of Von Buch, and maintaining that the great mountain chains of Europe are but the remnants of continental elevations which have been cut away by denudation, and that the foldings and inver- sions to be met W\i\i in the structure of mountains are to be looked upon only as local and accidental. In 1856 Mr. J. P. Lesley published a little volume entitled Coal and its Topography, (12 mo. pp. 224,) in the second part of which he has, in a few brilliant and profound chapters, discussed the prin- ciples of topographical science with the pen of a master. Here he tells us that the mountain lies at the base of all topographical geology. Continents are but congeries of mountains, or rather the latter are but fragments of coutineuts, separated by valleys which represent the absence or removal of mountain land [p. 126] ; and again " mountains terminate where the rocks thin out '' (p. 144.) The arrangement of the sedimentary strata of which mountains are composed may be either horizontal, synclinal, anticlinal or vertical, but from the greater action of diluvial forces upon anti- clinals in disturbed strata it results that great mountain chains are generally synclinal in their structure, being in fact but fragments of the upper portion of the earth's crust, lying in synclinals, and thus preserved from the destruction and translation which have exposed the lower strata in the ' ' 'clinal valleys, leaving the intermediate 20 Mr. T, Sterry Hunt mountains capped with lower strata. The effects of those great and mysterious denuding forces which have so powerfully modi- fied the surface of the globe become less apparent as we approach the equatorial regions, and accordingly we find that in the south- ern portions of the Appalachian chain many of the anticlinal folds have escaped erosion, and appear as hills of an anticlinal structure. The same thing is occasionally met with further north ; thus Sutton mountain in Canada, lying between two anticlinal valleys, has an anticlinal centre, with two synclinals on its opposite slopes. Its form appears to result from three anticlinals, the middle one of which has to a great extent escaped denudation. The error of the prevailing ideas upon the nature of mountain chains may be traced to the notion that a disturbed condition of the rocky strata is not only essential to the structure of a mountain, but an evidence of its having been formed by local upheaval, and the great merit of De Montlosier and Lesley, (the latter altogether independently,) is to have seen that the upheaval has been in all cases not local but conti- nental, and that the disturbance so often seen in the strata is neither dependent upon elevation nor essential to the forma- tion of a mountain. The synclinal structure of portions of the Alps, previously observed by Studer and others, has been beautifully illustrated by Ruskin in the fourth volume of his Modern Painters and in a late review of Alpine geology we have endeavoured to show that the Alps, as a whole, have likewise a synclinal structure. (Am. Jour. Science, xxix. 118.) Such was the state of the question when Mr. Hall came forward bringing his great knowledge of the sedimentary formations of North America to bear upon the theory of continents and moun- tains. These were first advanced in his address delivered before the American Association for the Advancement of Science, as its president, at Montreal in August, 1857. This address was never published, but the author's views were brought forward in the first volume of his Eeport on the Geology of Iowa, p. 41, and with more detail in the introduction to the third volume of his Paleon- tology of New York, from which we have taken the abstract already given. He has shown that the difference between the geographical features of the eastern and central parts of North America is directly connected with the greater accumulation of sediment along the Appalachians. He has further shewn that so far from local elevation being concerned in the formation of these i / on American Geology, Jj raonntains, the strata which form their base are to be found beneath their foundations at a much lower horizon than in the undisturbed hills of the Mississippi valley, and that to this depres- sion chiefly is due the fact that the mountains of the Ai)palachian range do not, like those hills, exhibit in their vertical height above the sea the whole accumulated thickness of the palaeozoic strata which lie buried beneath their summits. Mr. Hall has made a beautiful application of these views to explain the fact of the height of the Green Mountains over the Laurentides, and of the White Mountains over the former, by remarking that we have successively the Lower and Upper Silurian strata superimposed on those of the Laurentian system. The same thing is strikingly shown in the fact that the higher mountain chains of the globe are composed of newer formations, and that the summits of the Alps are probably altered sediments of tertiary age. (Am. Jour. Sci. xxix. 118.) The lines of mountain elevation of Do Beaumont are according to Hall, simply those of original accumulations, which took place along current or shore lines, and have subsequently, by continental elevations, produced mountain chains. " They were not then due to a later action upon the earth's crust, but the course of the chain and the source of the materials were predetermined by forces in operation long anterior to the existence of the mountains or of the continent of which they form a part." p. 86. It will be seen from what we have said of Buflfon, De Montlosier and Lesley that many of the views of Mr. Hall are not new but old; it was, however, reserved to him to complete the the- ory and give to the world a rational system of orographic geo- logy. He modestly says, " I believe I have controverted no established fact or principle beyond that of denying the influence of local elevating forces, and the intrusion of ancient or plutonio formations beneath the lines of mountains, as oMinarily understood and advocated. In this I believe I am only going back to the views which were long since entertained by geologists relative to continental elevations." p. 82. The nature of the palaeozoic sediments of North America clear- ly shows that they were accumulated during a slow progressive subsidence of the ocean's bed, lasting through the palaeozoic per- iod, and this subsidence, which would be greatest along the line of greatest accnmnlafinn wno flriiiKflaoo 0= Af.. tTqH -.__.-• i.„ o J " ^^ ••-"--) "c iiii. j_i.aii ^junsiuers, con- nected with the transfer of sediment and the variations of local pres- S3 Mr, T, Sierry Hunt sure Rctinaf upon tho yielding crust of the earth, agreeably to the view of Sir John Herschul. This subsidence of tho ocean's bottom would, according to Mr. Hall, cause plications in the soft and yield- ing Htrata. Lyell had already in speculating upon the results of a cooling and contracting sea of molten matter, huoIi as he imagined might have once underlaid the Appalachians, suggested that the in- cumbent flexible strata, collapsing in obedience to gravity would be forced, if this contraction took place along narrow and paral- lel zones of country, to fold into a smaller space as they con- formed to tho circumference of a smaller arc, " thus enabling tho force of gravity, though originally exerted vertically, to bend and Bqueeze tho rocks as if they had been subjected to lateral pressure * Admitting thus Herschel's theory of subsidence and Lyell's of plication, Mr. Hall proceeds to inquire into the great system of foldings presented by the Appalachians. The sinking along the line of greatest accumulation produces a vast synclinal, whrch is that of tho mountain ranges, and the result of such a sinking of flexible beds will be the production within the greater synclinal of numerous smaller synclinal and anticlinal axesrwhich must gradually decline toward the margin of the great synclinal axis. This process the author observes appears to furnish a satis- factory explanation of the difference of slope on the two sides of the Appalachian anticlinals, where the dips on one side are uni- formly steeper than on tho other, p. 71. An important question here arises, which is this ;— while admit- ting with Lyell and Hall that parallel foldings may be the result of the subsidence which accompanied the deposition of the Appal- achian sediments, we inquire whether the cause is adequate to produce the vast and repeated flexures presented by the AUe- ghanies. Mr. Billings in a recent paper in the Canadian Natu- ralist (Jan. 1860), has endeavoured to show that the foldinrrs thus produced must be insignificant when compared with thegreai > iu- lations of strata, whose origin Prof. Rogers has endeavor >hi :o ex- plain by his theory of earthquake waves propagated through the igneous fluid mass of the globe, and rolling up the flexible crust. We shall not stop to discuss this theory, but call attention to another agency hitherto overlooked, which must also cause contraction and folding of tb'.- f^ivaia, and to which we have already alluded. (Am. Jour.Sci.(2^s.xx. > 38.) It is the condensation which must take place when porous Fediments are converted into crystalline rocks like * Travels in North America, 1st visit, vol. i. p. 78. V I // ^' \,*' on American Geology, ts V I // § gncMis and rafoft slate, and still more when the elements of these sediments are changed into minerals of high speciflc gravity, such as pyroxene, garnet, epidotc, staurotido, ohiastolite and chloritoid. This contraction can o!i!y take place when the sediments have bo- come deeply buried and are undergoing raetamorphisra, and is, as many attendant phenomena indicate, connected with a softened and yielding condition of the lower strata. We have now in this connection to consider the hypothesis which ascribes the corrugation of portions of the earth's crust to the gradual contraction of the interior. An able discussion of this view will be found in the American Journal of Science (2) iii. 176, from the pen of Mr. J. D. Dana, who, in common with all others who have hitherto written on the subject, adopts the notion of the igneous fluidity of the earth's interior. We have however elsewhere given our reasona for accepting the conclusion of Hopkins and Hennessy that the earth, instead of being a liquid mass covered with a thin crust, is essentially solid to a great depth, if not indeed to the centre, so that the volcanic and igneous phenomena generally ascribed to a fluid nucleus have their seat, as Keferstein and after him Sir John Herschel long since suggested, not in the anhydrous solid unstratified nucleus, but in the deeply buried layera of aqueous sediments which, per- meated with water, and raised to a high temperature, become reduced to a state of more or less complete igneo-aqueous fusion. So that beneath the outer crust of sediments, and surrounding the solid nucleus, we may suppose a zone of plastic sedimentary material adequate to explain all the phenomena hitherto ascribed to a fluid nucleus. (Quar. Jour. Geol. Society, Nov. 1859. Canadian Naturalist, Dec. 1859, and Amer. Jour. Sci.(2)xxx. 136.) This hypothesis, as we have endeavoured to show, is not only completely conformable with what we know of the behaviour of aqueous sediments impregnated with water and exposed to a high temperature, but off'ers a ready explanation of all the phenomena of volcanos and igneous rocks, while avoiding the many difficulties which beset the hypothesis of a nucleus in a state of igneous fluidity. At the same time any changes in volume resulting from the contraction of the nucleus would aflfect the outer crust through the medium of the more or less plastic zone of sediments, precisely as if the whole interior of the globe were in a liquid state. The accumulation of a great thickness of sediment along a 24 Mr» T» Sterry Hunt I » given line would, by destroying the equilibrium of pressure, cause the somewhftt flexible crust to subside; the lowt.r strata beco'^'^'g altered by the ;scending heat of the nucleus would crystallize and contract, and plications would thus be determined parallel to \h"> line of deposition. These foldings, not le?s than the softening of the bottom strata, establish lines of weakness oi of least resistance in the earth's crust, and thus determine tho contraction which resultis from the cooling of the globe to exhibit itself in those regions and along those lines where the oce{»n's bod is subsiding beneath the accumulating sediments. Hence we conceive tbat the subsi- dence invoked by Mr. Ilall, although not the sole nor even the principal cause of the corrugations of the strata, is the one which determines their position and direction, by making the effects pro- duced by the contraction not only of sediments, but of the earth's nucleus itself, to be exerted along the lines of greatest accumulation. It will readily be seen that the lateral pressure vvhich is brought to bear tpon the strata of an elongated basin by the contraction of the globe, would cause the folds on either side to incline to the margin of the basin, and hence we find along the Appalachians, which 03cupy the western side of such a great synclinal, the steeper slopes, the overturn dips or folded flexures, and the overlaps from dislocation are to the westward, so that the general dip of the strata is to the centre of the basin, on the other side of which we might expect to find the reverse order of dips pre- vailing. The apparent exceptions to this order of upthrows to the south-east in the Appalachians appear to be due to small downthrows to the south-east, which are parallel to and immedi- ately to the north-west of great upheavals in the same direction. Mr. Hall adopts the theory of metamorphisra which we have expounded in the paper just quoted above, Canadian Naturalist^ Dec. 1859, (see also Am. Jour. Sci. (2) xxv. 287, 435, xxx. 135,) which has received a strong confirmation from the late researches of Daubree. According to this view, which is essentially that put forward by Herschel and Babbage, these changes have been eff'ected in deeply buried sediments by chemical reactions, which we have endeavored to explain, so that metamorphism, like folding, takes place along the lines of great accumulation. The appearance at the surface of the altered strata is the evidence of a considerable denudation. It is probable that the gneissic rocks of Lower Silu- rian age in North America were at the time of their crystalliza- iion overlaid by the whole of the palxozoic strata, while the \ on American Geology. 25 \ metamorphism of carboniferous strata in eastern New England points to the former existence of great deposits of newer and over- lying deposits, which were subsequently swept away. On the subject of igneous rocks and volcanic phenomena, Mr. Hall insists upon the principles which we were, so far as we know .the first to point out, namely their connection with great accumula- tions of sediment, and of active volcanos with the newer deposits. We have elsewhere said : " the volcanic phenomena of the present day appear, so far as are aware, to be confined to regions of newer secondary a.id tertiary deposits, which we may suppose the central heat to be still penetrating, (as shewn by Mr. Babbage,) a process which has long since ceased in the palaeozoic regions." To the accumulation of sediments then we referred both modern volcanos and ancient plutonic rocks ; these latter, like lavas, we regard in all cases as but altered and displaced sediments, for which reason we have called them exotic rocks. (Am. Jour. Sci. (2) xxK. 133). Mr. Hall reiterates these views , and calls attention moreover to the i"aGc that the greatest outbursts of igneous rock in the various formations appear to be in all cases connected with rapid accumulation over limited areas, causing perhaps disruptions of the crust, through which the semi-fluid stratum may have risen to the surface. He cites in this connection the traps with the palaeozoic sandstones of Lake Superior, and with the mesozoic sandstones of Nova Scotia and tLe Connecticut and Hudson valleys. It may sometimes happen that the displaced and liquified sub- stratum will find vent, not along the line o" greatest accumulation, but along the outskirts of the basin. Thus in eastern Canada it is not along the chain of the Notre Dame mountains, but on the north-west side of it that we meet with the great outbursts of trachyte and dolerite, whose composition and distribution we have elsewhere described. (Report of Geological Survey for 1858, and Am. Jour. Science, (2) xxix. 285.) The Nortti American continent, from the grand simplicity of its geological structure and from the absence, over great areas, of the more recent formations, off'ers peculiar facilities for the solution of some of the great problems of geology ; and we cannot finish this article without congratulating ourselves upon the great progress in this direction which has been made within the last few years by the labors of American geologists. Montreal. March 1, 1861.