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Show 86 ISLA~D LIFE. (PART I. It is there that nature has always strewn the dust of continents to be." The Movements of Continents.-As we find these stratified rocks of different periods spread over almost the whole surface of existing continents where not occupied by igneous or metamorphic rocks, it follows that at one period or another each part of the continent has been under the sea, but at the same time not far .from the shore. Geologists now recognise two kinds of movements by which the depositc;; so formed have been elevated into dry land-in the one case the stra.ta remain almost level and undisturbed, in · the other they are contorted and crumpled, often to an enormous extent. The former often prevails in plains and plateaus, while the latter is almost always found in the great mountain ranges. We are thus led to picture the land of the globe as a flexible area in a state of slow but incessant change ; the changes consisting of low undulations which creep over the surface so as to elevate and depress limited portions in succession without perceptibly affecting their nearly horizontal position, and also of intense lateral compression, supposed to be produced by partial subsidence along certain lines of weakness in the earth's crust, the effect of which is to crumple the strata and force up certain areas in great aontorted masses, which, wheu carved out by subaerial denudation into peaks and valleys, constitute our great mountain systems.l In this way every part of a continent may again and again have sunk beneath the sea, and yet as a whole may never have ceased to exist as a continent or a vast continental archipelago. • Professor Dana points out that the regions which, after long undergoing subsidence, and accumulating· vast piles of sedimentary deposits, havo been elevated into mountain ranges, have thereby become stiff and unyielding, and that the next depression and subsequent upheaval will be situated on one or the other sides of it; and he shows that, in North America, this is the case with all the mountains of the successive geological formations. Thus, depressions and elevations of extreme slowness but often of vast amount, have occurred successively in restricted adjacent areas ; and the effect has been to bring each portion in succession beneath the ocean but always bordered on one or both sides by the remainder of the continent, from the denudation of which the deposits are formed which, on the subsequent upheaval, become mountain ranges. (Manual of Geology, 2nd Ed., p. 751.) cHAr. v1.] GEOGRAPHICAL AND GI~OLOGICAL CHANGES. 87 And, as subsidence will always be accompanied by deposition, piles of marine strata many thousand feet thick may have been formed in a sea which was never very deep, by moans of a slow depression either continuous or intermittent, or through alternate subsidences and elevations, each of moderate amount. Supposed Oceanic .Formations ,·-the Origin of Challc.-There seems very good reason to believe that few, if any, of the rocks known to geologists correspond exactly to the deposits now forming at the bottom of our great oceans. The white oceanic mud, or Globigerina-oozc, found in all the great oceans at depths varying from 250 to nearly 3,000 fathoms, and almost constantly in depths under 2,000 fathoms, has, however, been supposed to be an exception, and to correspond exactly to our white and grey chalk. Hence some naturalists have maintained that there has probably been one continuous formation of chalk in the Atlantic from the Cretaceous epoch to the present day. This view has been adopted chiefly on account of the similarity of the minu'te organisms found to compose a considerable portion of both deposits, more especially the pelagic Foraminifera, of which several species of Globigerina appear to be identical in the chalk and the modern Atlantic mud. Other extremely minute organisms whose nature is doubtful, called coccoliths and discoliths, are also found in both formations, while there is a considerable general resemblance between tho higher forms of life. Sir W yville Thomson tolls us, that" Sponges are abundant in both, and the recent chalk-muu has yielded o. large number of examples of the group porifera vitrea, whjclt find their nearest representatives among the Ventriculites of the white chalk. The echinoderm fauna of the deeper parts of the Atlantic basin is very characteristic, and yields an assemblage of forms which represent in a remarkable degree the corresponding group in the white chalk. SpecieR of the genus Cidaris are numerous; some remarkable flexible forms of the Diac.lemidru seem to approach Echinothuria." 1 Now as some explanation of the origin of chalk had long been desired by geologists, it is not surprising that the amount of resemblance shown to exist between it and some kinds of 1 Natw·e, Vol. II., p. 297. |
