Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/384

Rh 3 70 the White Mountains. the Rocky Mountain slopes, vhere it might have been looked for, is remarkable. Underneath the boulder-clay the solid rocks, as in Europe, are often well striated. The direction of the stria: is generally south- wm-.l, varying to south-ea.st and south-west according to the form of the ground. In recent _vears extensive icc-worn rock-surfaces have been obse1'vcd among the I{ock_' Mountains by llaydcn, King, and others, proving that these elevations formerly possessed their glaciers, if they were not buried under the great ice-sheet. 'l‘be drift bears witness to a general southerly transport of material, and, in conjunction with the striated rocks, shows that the great ice- shect moved from north to south at least as far as about the latitude of Vashington. Logan mentions that in some parts of Canada the glacial drift and boulders run in ridges north and south, thus corre- sponding with the general direction of trans ort, like the “ drums " in Britain. As in Europe, the coarse bou der-clay at the base of the Quaternar_y deposits is essentially unfossilifcrous. 2. C'lm.mplain.—Undcr this name .meriean geologists class the sands, gravels, and clays vhich overlie the lover boulder-drift. These deposits include coarse unstratificd gravel:s, as well as finely- stratificd cla_vs. In eastern Canada they are well developed, and show the following subdivisions :— St Maurice and .'orcl sands; Sarz'cara sand of lIontreal; Upper. upper sand and gravel of Beauport; upper Champlain clay and sand of Vermont. I0“_m_ i Leda clay of the St Lawrence and Ottawa; lover shell- ‘ ‘ I sand of Bcauport; lower Champlain clay of Vermont. T he lover subdivisions consist chiefly of clays, which rise to a height of 600 feet above the sea. They have some intcrstratified beds of siliceous sand, but few boulders. They contain marine organisms, su:h as Lula Iruncatu, .S'a.1:icaz'a rugosa, Tcllina Groan- Immfica, bones of seals, whales, &c. On the banks of the Ottawa, in Gloucester, the clays contain numerous nodules which have been formed round organic bodies, particularly the ﬁsh Jiallolus rillosus or ca eliue of the Lower St Lawrence. Dawson also obtained HllH1f‘II‘)Ol1S cremains of terrestrial marsh plants, grasses, cariccs, mosses, and algae. This writer states that about 100 species of marine invertebrates lliave been obtained from the clays of the St Lawrence Valle . Al except four or ﬁve species in the oh er mrt of the deposihly are shells of the boreal or Arctic regions of [the Atlantic; and about half are found also in the glacial clays of Britain. The great majority are now living in the Gulf of St Lawrence and neighbouring coasts, especially off Labrador.‘ 3. Tcrracc.—'l‘hi:s division includes the terraced deposits of alluvial matc.rial so marked along the river valleys and lake margins in the northern part of the United States and in Canada, and found also in some degree along the sea-coast These deposits occur in successive platforms or terraces, marking the contraction in volume of the lakes and rivers, consequent, probably, upon intermittent up- heavals of the land. They are well (levelo wed round the great lakes. Thus in the basin of Lake Huron depositsl of fine sand aiill clay con- taining fresh-watcr shells rise to a height of 40 feet or more above the presenp level of the water, and run back from the shore sometimes for 20 mi es. Reoular terraces, corrcs iondinrr to former water- lcvels of the lake, lfllll for miles along thle sliorels at heights of 120, l:')0, and 200 feet. Shingle beaches and mounds or ridges, exactly like those now in course of formation along the e.' )0S(3(l shores of Lake Huron, can be recognized at heights of 60, 701, and 100 feet. Unfossiliferous terraces occur abundantly on the margin of Lake Superior. At one point mentioned by Logan, no fewer than seven c-f these ancient b -aches occur at intervals up to a height of 331 feet above the present level of the lake.“ Most of the rivers are bordered with lines of terraces, as in the 'cll-known example of the Con- necticut valley described by llitchcock. The rivers are believed to have had their maximum volume at the beginning of the Terrace epoch, swollen doubtless by the melting of the still existing ire- .~ln-etsnand snow-lfields. 'l‘lnfir}prk1 colnsisted partly in depositing fine a uviuni or oess over t ieir oo: - ) ains, Xlftl 'in scourin their channels out of the Champlain fotimationls. )Greatcr clfvation towards the interior, by augmenting their slope, increased their -xcavating power. Terraces of marine origin likcwisc occur both on the coast and far inland. On the coast of Maine they occur at heights of 150 to 209 feet, round Lake Champlain at least as high as 300 feet, and at Montreal nearly 500 feet above the present level of the sea. In the absence of organic remains, however, it is not always possible to distinguish between terraces of marine origin marking former sea- margins, and those left by the retirement of rivers and lakes. In the Bay of Fundy evidence has been cited by Dawson to prove sub- --idence, for he has observed there a submerged forest of pine and l.-_""(‘ll lying 23 feet below high—water mark.3 -4 Ifvrcnt am? I’:-(I11'storz°c. —Thc deposits in this group are essen- ‘ .lca¢li(zn 6'col0_r/_.;, p, 71?. 2 C-‘eulogy If {'unrula', p. 910. ’ Acmlian (.o'eoln_/y. p. 23. GEOLOGY The absence of any true boulder-drift on I tially the s:nnc with those in Europe; and, as in that continent so [vn. I-Hvsroe.RAru1cAL. in America. no definite lines can be dravn within which they should be confined. 'l‘hcy cannot be sharply separated from the Terrace series, on the one hand, nor from modern accumulations, on tha- other. lcsides the marshes, peat-bogs, and other organic deposits which belong to an early period in the human occupation of America, some of the younger alluvia of the rivcr—valleys and lakc.s can no doubt claim a. high antiquity, though they have not supplied the samc copious evidence of early man which gives so much interest to the corresponding Curopcan formations. Heaps of shells of edible species occur on the coasts of N ova Scotia, Maine, &c. The large mounds of artificial origin in the Mississippi valley have c.'cited much attention. PART VII.-—PIlYSIOG lA1‘IIIC-L GEOLOGY. In the investigation of the geological history of any country, two questions present themselves. We have first to consider the nature and arrangement of the rocks which underlie the surface, and to ascertain from them what lms been the march of events, what changes in geography have successively taken place, and what races of plants and animals have come and gone. The gradual geological evolution of the earth has been sketched in the foregoing part of this article. But besides the history of the solid rocks beneath the surface of the land, there is that of the surface itself. Mountains and plains, valleys and ravines, cliffs, peaks, passes, lakes, and the many other features of a country demand attention. By what processes have these varied outlines been impressed upon the surface of the globe? Are they of different ages, and if so, how can their history be ascertained? The branch of geological inquiry which endeavours to answer these questions has been termed Physiography or Physiographical Geology. Its investigations evidently de- mand an acquaintance with Stratigraphical (_iCnl0g‘y. We must be able to trace out the former geographical conditions of the globe before we can adequately reason on the origin of those now existing. Hence the consideration of this branch of the subject has necessarily been reserved for tllls concluding section. The stratified fortnations, of which the succession and history have been traced in the previous pages, were chiefly laid down on the sca—floor in wide horizontal or gently inclined sheets. They have since been upraised into land; their horizontality has been in great part destroyed; and they have been enormously wasted by dcnuding agents. In con- sidering therefore how they have acquired their present ex- ternal forms, we have to deal with the eﬂ'ects of two kintls of forces, one acting from below, the other on the surface. These stratified rocks were, on the whole, deposited in shallow water, and have been repeatedly 1lpl‘U.l.<cLl and denuded, so that the younger have been formed out of the waste of the older. They have their modern counterparts, 11ot in the deposits of the great oceau—basins, but in those. of coniparativcly shallow seas. The inference to be drawn from these facts is that the present continental region.-', through many local oscillations, have existed as terrestrial ridges from a remote geological antiquity, and that the ocean basins in like manner have, on the whole, retained their identity. 'hen the geologist asks himself how the present distribution of sea and land is to be accounted for, he finds that the answer to the question goes back to earl y Palmozoic times, whence he can in some cases trace the gradual growth of a continent downward through the long cycles of geological time. But there still remains the problem to account for the original wrinkling of the surface of the globe, I whereby the present great ridges and hollows were produced. It is now generally agreed that these inequalities have been produced by unequal contraction of idle earth’s lll{1.~‘.'', the interior contracting more than the outer crust, which must therefore have accommodated itself to this diminution of diameter by undergoing corrugation. But there seems