12 AN INTRODUCTION TO THE STUDY OF FOSSILS 



many other states. In the case of plants, the wood may 

 act as a sponge, capillary attraction drawing up the siliceous 

 water, while possibly the high density of this liquid would aid 

 the solution and extrusion of the cellulose (woody fiber). 



This molecular replace- 

 ment is most common 

 with silica, lime and iron. 

 In iron, replacement takes 

 place in the presence of 

 organic substances; per- 

 colating waters rich in iron 

 and sulfur will be precipi- 

 tated upon coming into 

 contact with these, caus- 

 ing the formation of mar- 

 casite or of iron pyrites 

 (FeS2) (Fig. 2). 



Silica is soluble in 

 waters containing carbon- 

 ates of the alkalis (Na, K) or alkaline earths (Ca, Mg), such as 

 the ordinary water leaching through the soil. This solution 

 (alkaline solvent of silica), coming in contact with the carbon 

 dioxid of a decaying organism, is neutralized and the silica 

 thrown down ; or this precipitation of silica may occur through 

 the agency of the carbon dioxid evolved by a living organism, 

 such as the diatom, which thus manufactures its siliceous skele- 

 ton. The siliceous skeleton of organisms is composed of hydrous 

 silica and is subject to dehydration into chalcedony, or to a 

 later crystallization into quartz. Hence deposits of the minute 

 diatom skeletons (about 40 miUion to the cubic inch), known 

 as tripolite, are rarely older than the Tertiary, since the older 

 deposits have probably been dissolved and redeposited as chert. 

 Similar replacement by silica occurs very frequently at the 

 surface of a rock ; probably the majority of silicified shells are 

 thus formed. 



Fig. 2. — Iron replacing lime in the Devonian 

 coral, Heliophyllum halli E & H, from New 

 York. Natural size. The iron forms the 

 ramifying white Unes upon each side ; this 

 largely follows the septa, but it branches 

 from these in all directions. 



