LIFE : ITS MECHANICAL WORK AND ROCK CONTRIBUTIONS. 143 



of this kind are not common. Siliceous 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 decomposition, or an altera- 

 tion, through chemical means, changing the texture of the fossil, or petrify- 

 ing it, as in the turning of wood into stone. 



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

 complete loss of the decomposable animal portion of the bone or shell, a process that leaves 

 shells and bones fragile. It may be a loss of part of the mineral ingredients by solvent 

 waters, as of the phosphates and fluorides 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 combin- 

 ing of the old elements into new compounds, as when a plant decays and changes to coal or 

 one or more carbohydrogens, a resin to amber, animal matter to adipocere. It may be 

 merely one of crystallization. 



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 oxide of iron. Through 

 this method bones may become as firm as when living, though also much heavier. 



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 subjected to the action of waters con- 

 taining silica in solution. In other cases, the organism becomes changed to calcium car- 

 bonate, as in much petrified wood ; and in others, to oxide of iron, or to pyrite ; and more 

 rarely to fluor spar, barite, or apatite. 



The mineral matter first fills the cells of the wood, and then takes the place of each 

 particle as it decomposes and passes away, until finally the original material is all gone. 

 Some fossil logs are carbonized at one end and silicified at the other. 



The silica in most siliceous petrifactions has come from siliceous organisms, such 

 as species of Sponges, or shells of Diatoms, from living species of the period that were 

 associated with the fossil in the original deposit. 



exaiyiples of the formation of strata through the 



Agency of Life. 



1. Deposits from Pelagic and Abyssal Life. 



1. Plants. — Ordinary seaweeds, although in general littoral species, float 

 widely over the ocean in some seas, as in the case of the Sargasso Sea of 

 the north Atlantic. Moreover, the shore seaweeds are often drifted off by 

 the currents. But the supply, while of importance as food for the animal 

 species of the sea-bottom, makes no abyssal vegetable deposits. Dredging 

 has brought up no remains of such deposits. 



But Diatoms, which becloud the waters of the southern ocean, and there 

 serve for the vegetable food of Whales, make the great deposits of Diatom 

 ooze, as already described, besides giving a sprinkling of siliceous shells over 

 all other parts of the ocean's bottom. These shells, as stated by Murray, 

 are especially noticeable in the deeper Red ooze, because the carbonic acid, 

 which removes calcareous relics, leaves them uninjured. 



