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Show 98 ISLAND LIFE. [PART 1. continents nor continental islands existed ~here our oceans now extend ; for had they existed, P::tlceozoic and Secondary formations would in all probability have been accumulated from sediment derived from their wear and tear; an.d t~ese would have been at least partially upheaved by the osCillatiOns of level which must have intervened during these enormously long pe;iods. If then we may infer anything from these facts, we may infer that, where our oceans now extend, oceans have extended from the remotest period of which we have any record; and, on the other hand, that where continents now exist larcre tracts of land have existed, subjected no doubt to grea~ oscillations of level, ~ince the Cam.brian period." . Th~s arO'ument standinO' by itself has not received the attention 1t de~erves, but coming in support of the long series of facts of an altoO'ether distinct nature, going to show the permanence of contine~ts, the cumulative effect of the whole must, I think, be admitted to be irresistible. 1 1 Of late it has been the custom to quote the so-called "ridge" down the centre of the Atlantic as indicating an extensive ancient land. E~ren Professor Judd adopts this view, for he speaks of the great belt of Tertiary ,,olcanoes "which extended through Greenland, Iceland, the Faroe Islands, the Hebrides, Ireland, Central France, the Iberian Penins~la, the Azores, Madeira, Canaries, Cape de Verde Islands, Ascension, tit. Helena, .and Tristan d' Acunha and which constituted as shown by the recent sound1~gs of H. M S. Challe1~ge1· a mountain-range, comparable in its extent, elevati.on, and volcanic character with the Andes of South American . ( Geologtc~l :Mag. 1874, p. 71). On examining the diagram of the ~tla~tiC ?cean m the Ohallenge1· Re1101·ts No. 7 a considerable part of this ndge IS found to be more than 1,9'J0:' 0 fat' homs 'd eep, while the portion cal 1e dtl 1e "Co nnec t.'m g Ridge " seems to be due in part to the deposits CaiTi~d out by the 1-h.ver Amazon. In the neighbourhood of the Azores, St. Pauls Rocks, AscensiOn, and Tristan d'Acunh~t are considerable areas varying from 1,200 to 1,500 fathoms deep, while the rest of the ridge is usually 1,800 or 1,900 fathoms. The shallower water is no doubt due to volcanic upheaval and the accumulation of volcanic ejections, and there may be many other deeply submer~ed old volcanoes on the ridge; but that it ever formed a chain of mot~ntams '' co~nparable in elevation with the Andes," there se~ms. not ~ p.arbcle of evidence to prove. It is however probable th~t .tlus ndge .mdw~tes the former existence of some considerable Atlantic Islands, winch Will serve to explain the presence of a few identical ge~era, an~ even sp~cies of plants and insects in Africa and South Amenca,. while. the m~1~ bocly of the fauna and flora of these two continents remams ntd1cally d1otmct. CHAP. VI.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 99 General Stability of Continents with Constant Change of Fo?'?n. -It will be observed that the very same evidence which has been adduced to prove the general stability and permanence of our continental areas also goes to prove that they have been subjected to wonderful and repeated changes in detail. Every square mile of their surface has been aO'ain and aO'ain under b b water, sometimes a few hundred feet deep, sometimes perhaps several thousands. Lakes and inland seas have been formed have been filled up with sediment, and been subsequently raiseci into hills or even mountains. Arms of the sea have existed crossing the continents in various directions, and thus completely isolating the divided portions for varying intervals. Seas have been changed into deserts and deserts into seas. Volcanoes have grown into mountains, have been degraded and sunk beneath the ocean, have been covered with sedimentary deposits, and again raised up into mountain ranges; while other mountains have been formed by the upraised toral reefs of inland seas. The mountains of one period have disappeared by denudation or subsidence, while the mountains of the succeeding period have been rising from beneath the waves. The valleys, the ravines, and the mountain peaks, have been carved ont and filled up again; and all the vegetable forms which clothe the earth and furnish food for the various classes of animals have been completely changed again and again. Effect of Continental Changes on the Distribuiion of Animals.It is impossible to exaggerate, or even adequately to conceive, the effect of these endless mutations on the animal world. Slowly but surely the whole population: of living things must have been driven backward and forward. from east to west, or from north to south, from one side of a continent or a hemisphere to the other. Owing to the remarkable continuity of all the land masses, animals and plants must have often been compelled to migrate into other continent , where in the struggle for existence under new conditions many would succumb; while such as were able to survive would constitute those wide-spread groups whose distribution often puzzles ns. Owing to the repeated isolation of portions of continents for long periods, special forms of life would. have time to be II 2 |
