Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/334

Rh 320 trees, herbage, and the bodies of land animals, the remains so transported run every risk of decaying or being other- ' wise destroyed while still afloat. and that even if they reach the bottom they will tend to dissolution there unless speedily covered up and protected by fresh sediment, we must perceive that delta formations can scarcely be ex- pected to give us more than a meagre outline of the varied terrestrial ﬂora and fauna. 4. Cat-erns.—These are eminently adapted for the pre- servation of the higher forms of terrestrial life. Most of our knowledge of the prehistoric mammalian fauna of Europe is derived from what has been disinterred from ban-9-caves. As these recesses lie for the most part in lime- stone or in calcareous rock, their floors are commonly coated with stalagmite from the drip of the roof ; and as this de- posit is of great closeness and durability it has effectually preserved whatever it has covered or enveloped. The caves have in many instances served predatory beasts, like the hyaena, cave-lion, and cave-bear, as dens in which they slept, and into which some of them dragged their prey. In other cases they have been merely holes into which different animals crawled to die, or into which they fell or were swept by inundations. Under whatever circum- stances the animals left their remains in these subter- ranean retreats, the result has been that the bones have been covered up and preserved. Still, we nmst admit that after all but a mere fraction even of the mammals of the time would enter the caves, and therefore that the evi- dence of the cavern-deposits, profoundly interesting and valuable as it is, presents us with merely a glimpse of one aspect of the life of the land. II. In the next place, if we turn to the sea, we ﬁnd cer- tainly many more favourable conditions for the preservation of organic forms, but also many circumstances which operate against it. While the level of the land remains stationary, there can be but little effective entombment of marine organisms in littoral deposits; for only a limited accumulation of sediment will be formed until subsidence of the sea-floor takes place. In the triﬂing beds of sand or gravel thrown up on a stationary shore, only the harder and more durable forms of life, such as gasteropods and lamellibranchs, which can withstand the triturating effects of the beach waves, are likely to remain uneffaced. Below tide-marks, along the margin of the land where sediment is gradually deposited, the conditions are favour- able for the preservation of marine organisms. Sheets of sand and mud are there laid down. In those sediments the harder parts of many forms of life may be entombed and protected from decay. But only a small proportion of the total marine fauna may be expected to occur in such deposits. At the best, merely littoral and shallow-water forms will occur, and even under the most favourable con- ditions they will represent but a fraction of the whole assem- blage of life in these juxta-terrestrial parts of the ocean. As we recede fron1 the land the rate of deposition of sedi- ment on the sea—ﬂoor must become feebler, until in the remote central abysses it reaches a hardly appreciable mini- mum. Except, therefore, where organic deposits, such as ooze, are forming in these more pelagic regions, the condi- tior_s must be on the whole unfavourable for the preserva- ticn of any adequate representation of the deep-sea fauna. Hard enduring objects, such as teeth and bones, may slowly accunmlate and be protected by a coating of peroxide of rarzganese, or of some of the silicates above (p. 288) referred to as now forming here and there over the deep- sea-bottom. But such a deposit, if raised into land, would .-gupply but a meagre picture of the life of the sea. We must conclude therefore that of the whole sea—ﬂoor the area best adapted for preserving a varied suite of marine organic exuvire is that belt which, running along GEOLOGY [v. mL.+:o.'roLoaIc. I. l the margin of the land, is ever receiving fresh layers of sediment transported by rivers and currents from the adjacent shores. The most favourable con(litions for the ' accumulation of a thick mass of marine fossilifcrous strata will arise when the area of deposit is undergoing a gradual subsidence. If the rate of depression and that of deposit were equal, or nearly so, the movement might proceed for a vast period without producing any great apparent. change in marine geography, and even without serious-ly affecting the distribution of life over the sea-floor within the area of subsidence. Hundreds or tliousamls of feet of sedi- mentary strata might in this way be heaped up round the continents, containing a fragmentary series of organic re- mains belonging to those forms of shallov.-water life which had hard parts capable of preservation. There can be little doubt that such l1a.s in fact been 1h.- history of the main mass of stratified formations in the earth’s crust. These piles of marine strata have unques- tionably been laid donn in comparatively shallow water within the area of deposit of terrestrial sediment. Their great depth seems only explicable by prolonged and repeat-. d movements of subsidence, interrupted, however, as we know, by other movements of a contrary kind. These geographical changes affected at once the deposition of inorganic mate- rials and the succession of organic forms. One series of strata is sometimes abruptly succeeded by another of a very different character, and we generally ﬁnd a corresponding contrast between their respective organic contents. It follows from these conclusions that representatives of the abyssal deposits of the central oceans are not likely to be met with among the geological formations of past times. Thanks to the great work done by the “ Challenger” expe- dition, we now know what are the leading cliaracters of these abyssal deposits of the present day. They have absolutely no analogy among the formations of the. earth's crust. They differ, indeed, so entirely fro1n any formation which geologists considered to be of deep-water origin as to indicate that, fron1 early geological times, the present great areas of land and sea have remained on the whole where they are, and that the land consists mainly of strata formed at successive epochs of terrestrial debris laid down in the surrounding shallow sea. Relative Value of Orgcmic I?em(u'n.s as I"oss1'ls. As the conditions for the preservation of organic remains exist more favourably under the sea than on land, marine organ- isms must be far more abundantly conserved than those of the land. This is true to-day, and has been true in all past geological time. Hence for the purposes of the geolo- gist the fossil remains of marine forms of life far surpass all others in value. Among them there will necessarily be a. gradation of importance regulated chieﬂy by their relative abundance. N ow, of all the marine tribes which live within the juxta-terrestrial belt of sedimentation, unquestionably the Jfollusca stand in the place of pre-eminence as regards their aptitude for becoming fossils. In the ﬁrst place they almost all possess a hard durable shell, capable of resisting considerable abrasion, and readily passing into a mineralized condition. In the next place they are extremely abundant both as to individuals and genera. They occur on the shore within tide mark, and range thence down into the abysses. Moreover, they appear to have possessed those qualiﬁcations from early geological times. In the marine Jfollusca, therefore, we have a common ground of compari- son between the stratiﬁed formations of different periods. They have been styled the alphabet of palueontological inquiry. It will be seen, as we proceed, how much in the interpretation of geological history depends upon the testi- mony of sea-shells. Looking at the organisms of the land, we perceive that, as a rule, the abundant terrestrial ﬂora has a comparatively