aK. sees 7 L 
a 4 
608 PALHONTOLOGICAL GEOLOGY. — [Boox V. 
in marine geography, and even without seriously affecting the 
distribution of life over the sea-floor within the area of subsidence. 
Hundreds or thousands of feet of sedimentary strata may conceivably 
be in this way heaped up round the continents, containing a frag- 
mentary series of remains, chiefly forms of shallow-water life which 
had hard parts capable of preservation. : 
There can be little doubt that such has in fact been the history of 
the main mass of stratified formations in the earth’s crust. These 
piles of marine strata have unquestionably been laid down in com- 
paratively shallow water within the area of deposit of _ terrestrial 
sediment. Their great depth seems only explicable by prolonged and 
repeated 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 materials and the suc- 
cession 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 
abysmal 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 expedition, we know what are 
the leading characters of the accumulations now forming on the 
deeper parts of the ocean floor. They have absolutely no analogy 
among the formations of the earth’s crust. They differ, indeed, so 
entirely from any formation which geologists have considered to be 
of deep-water origin as to indicate that, from 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. 
ii. Preservation of organic remains in mineral masses.— 
The condition of the remains of plants and animals in rock-forma- 
tions depends, first, upon the original structure and composition of 
the organisms, and secondly, upon the manner in which their 
fossilization has been effected. 
1. Influence of original structure and composition, 
—The internal skeletons of most vertebrate animals consist mainly of 
phosphate of lime. In saurians and fishes there is also an exo- 
skeleton of hard bony plates or of scales. It is these durable portions 
that remain as evidence of the former existence of vertebrate life. 
The hard parts of invertebrates present a greater variety of composition, 
In the vast majority of cases they consist of calcareous matter, either 
calcite or aragonite (pp. $2, 83). The carbonate of lime is 
occasionally strengthened by phosphate, while in a few cases, as in 
the horny brachiopods, in conularia, serpula, and some other forms, 
the phosphate is the chief constituent." Next in abundance to lime 
1 Logan and Hunt. Amer. Journ, Sci. xvii, (1854), p. 235, 
