Fig. 11 



FOSSILS: THEIR ORIGIN AND DISTRIBUTION. 193 



deoxidized by the organic matter, the latter passing off as carbonic acid 

 and water, and the former becomes insoluble sulphide (FeS), and is 

 deposited. Now, as each particle of organic matter passes away as 

 C0 2 and H 2 0, the molecule of iron sulphate which effected the change 

 is itself changed into insoluble sulphide, and takes its place. 



The process of replacement by silica (silicification) is less clear, but 

 it is probably as follows : Silica is found in solution in many waters, 

 being held in this condition by small quantities of alkali present in the 

 waters. In contact with decomposing wood the alkali is neutralized 

 by the humic, ulmic, and other acids of decomposition, and the silica 

 therefore deposited. 



3. Organic Form only "preserved. — In the third case organic mat- 

 ter and organic structure are both lost, and only organic form is pre- 

 served. This kind of 

 fossilization is most com- 

 monly seen in shells. It 

 may be subdivided into 

 four subordinate cases, 

 represented in section by 

 a, b, c, and d of Fig. 175. 

 In this figure the horizontal lines represent the original sediment which 

 may or may not have consolidated into rock ; the vertical lines repre 

 sent a subsequent filling of different and usually finer material. In a 

 we have a mold of the external form of the shell preserved in sediment. 

 The shell with the undecayed animal was imbedded, and afterward en- 

 tirely dissolved away, leaving only the hollow mold. In b the same 

 process has taken place, only the mold has been subsequently filled by 

 infiltration of slightly soluble matters. In this case we have both the 

 mold and the cast of the external form ; the mold being formed of sed- 

 iment, and the cast of infiltrated matter. These are always of different 

 materials, i. e., different either in chemical composition or in state of 

 aggregation. In c we have a mold of the external form in sediment, 

 and a cast of the internal form in the same material with an empty 

 space between, having the exact form and thickness of the shell. In 

 this case, the already dead and empty shell was imbedded in sediment, 

 ichich also filled its interior ; afterward the shell was removed, leaving 

 an empty space. In d this empty space was subsequently filled by 

 infiltration. In shore and river deposits of the present day it is very 

 common to find shells imbedded in, and filled with, sand or mud. In 

 the more recent tertiary rocks shells are commonly found in the same 

 condition precisely ; but in the older rocks more commonly the origi- 

 nal shell is removed, and the space either left empty or filled by infil- 

 tration. Cases c and d are well represented by Figs. 176, 177, and 178. 

 Cases like a and c are most commonly found in porous rocks like sand- 

 13 



