LIFE. 611 



tinued on into the age of Man, while as regards terrestrial animal life there were 

 in this interval many successive faunas. w 



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

 Crinoids, Mollusks and Rhizopods ; for the reason that only the sim- 

 plest 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 ; for all ani- 

 mals grow principally by the multiplication of cells ; and when single 

 cells or minute groups of them, as in the Rhizopods, are independ- 

 ent animals, the increase may still be the same in rate, or even 

 much more rapid, on account of the simplicity of structure. 



3. Methods of Fossilization and Concretion. 



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. Siliceous Dia- 

 toms 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 

 decomposition, or an alteration through chemical means, changing 

 the texture 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 compounds, as when a plant decays and 

 changes to coal and bitumen, 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 of calcite, as often happens in encrinal columns 

 and the spines of Echinoids. 



The change often consists in the reception of new mineral matter into the 

 pores or cellules of the fossil, as when bones are penetrated by limestone or oxyd 

 of iron. 



The change is frequently a true petrifaction, in which there is a substitution 

 of new mineral material for the original; as when a shell, coral or wood is 

 changed to a siliceous fossil through a process in which the organism was sub- 

 jected to the action of waters containing silica in solution. In other cases, the 

 organism becomes changed to carbonate of lime, as in much petrified wood ; 



