614 DYNAMICAL GEOLOGY. 



served, than those which frequent dry forests. But, whatever their habits, birds are 

 among the rarest of fossils, because they usually die on the land, are sought for as food 

 by numberless other species, and have slender hollow bones that are easily destro} r ed. 



Vertebrate animals, as fishes, reptiles, etc., which fall to pieces when the animal por- 

 tion is removed, require speedy burial after death, to escape destruction from this 

 source as well as from animals that would prey upon them. 



Fishes of the open ocean, having the means of easy locomotion through the waters, 

 would be less liable to destruction from changes of level in the land than the Mollusks 

 of a coast ; and hence some of the Sharks of the Tertiary continue through two or three 

 periods. 



The animals generally of the ocean are little liable to extermination from changes of 

 climate over the land; and hence some marine invertebrate species of the early Terti- 

 ary, many of the later, and all of the Quaternary, have continued on until now, while, 

 as regards terrestrial animal life, there have been in this interval many successive 

 faunas. 



(c.) The lowest species of life are the best rock-makers, especially Co- 

 rals, Crinoids, Mollusks, Rhizopods, Diatoms, and Coccoliths ; for the 

 reason that only the simplest kinds of life can be mostly of stone, and 

 still perform all their functions. Multiplication of bulk for bulk is 

 more rapid with the minute and simple species than with the higher 

 kinds ; fqr all animals grow principally by the multiplication of cells ; 

 and, when single cells or minute groups of them, as in the Rhizopods, 

 are independent animals, the increase may still be the same in rate per 

 cubic foot, or even much more rapid, on account of the simplicity of 

 structure. 



3. Methods of Fossilization and Accumulation. 



A. Fossilization. — In the simplest kind of fossilization, there is 

 merely a burial of the relic in earth or accumulating detritus, where it 

 undergoes no change. Examples of this kind are not common. Sili- 

 ceous Diatoms and flint implements are among them. 



In general, there is a change of some kind ; usually, either a loss, 

 by decomposition of the less enduring part of the organic relic, with 

 sometimes the forming of new products in the course of the decom- 

 position, or an alteration, through chemical means, changing the tex- 

 ture of the fossil, or petrifying it, as in the turning of wood into stone. 



The change may consist in a fading or blanching of the original colors ; in a partial 

 or complete loss of the decomposable animal portion of the bone or shell ; a similar loss 

 of part of the mineral ingredients, by solvent waters, as of the phosphates and fluorids 

 of a bone or shell ; or a general alteration of the original organism, leaving behind only 

 one or two ingredients of the whole; or a combining of the old elements into new com- 

 pounds, as when a plant decays and changes to coal or one or more carbobydrogens, a 

 resin to amber, animal matter to adipocere. 



The change may be merely one of crystallization. The carbonate of lime of shells is 

 often partly in the state of aragonite; and, when so, there is usually a change, in which 

 the whole becomes common or rhombohedral carbonate of lime (calcite). Sometimes, 

 the compact condition of the original fossil is altered to one with the perfect cleavage d 

 calcite, as often happens in the columns or plates of Crinoids and the spines of Echi- 

 noids. 



