INTRODUCTION II 



Fossils altered (petrifactions) . — Whenever organic remains 

 are inclosed in sediment to which the air has free access, their 

 disintegration is rapid and complete (page 5) ; but if the 

 deposits become very thick before they are raised from the 

 water, free air will be largely excluded and destruction of the 

 fossils will be principally due to the percolation of the water 

 through the strata from higher to lower levels. That 

 heat is not necessary in the solution of shells is seen in 

 many deposits of glacial age. For example, the drumlins in 

 and to the south of Boston Harbor, which have remained sur- 

 face deposits from the time of their formation, contain no shells 

 in the upper half, but below that level they are quite fossil- 

 iferous. Here it could only be the carbonic acid gas, caught 

 by the rain drops while falling through the air, that caused the 

 gradual solution of the shells. 



Silicification. — If the percolating w^ater is a supersaturated 

 solution of some mineral substance, such as silica or lime, the 

 first step in the petrifaction of the fossil will be the filling of all 

 existing cavities with this substance. In such a case the origi- 

 nal chemical composition of the shell, bone or wood will remain, 

 but with the many minute openings filled with the foreign min- 

 eral. This is the condition of many of the dinosaur and other 

 vertebrate bones of the Mesozoic. Often, however, the petri- 

 faction, or making into stone, is a process of solution and de- 

 position taking place pari passu, and due to the waters being 

 under-saturated with the chemical substances of which the fos- 

 sil is composed, but oversacurated with some other substance. 

 Such a replacement is at times very perfect. The probable pro- 

 cess is that as a molecule of the original structure is removed, 

 a molecule of the depositing substance takes its place ; in this 

 case the finer structure, such as the fiber in wood, or the pores 

 and lamellae in shells, is perfectly preserved. Such preservation 

 of the finer details of structure is seen in many of the sections 

 of the trunks of petrified trees from the fossil forests of the 

 Yellowstone National Park (Fig. 24), from Arizona and from 



