PART I. CHAPTER IV. 



55 



Mineralization of Organic Remains. 



forced against others previously consolidated, and, thus com- 

 pressed, may have acquired a new structure. 



But the action of heat at various depths in the earth is proba- 

 bly the most powerful of all causes in hardening sedimentary 

 strata. To this subject I shall refer again when treating of the 

 metamorphic rocks, and of the slaty and jointed structure. 



Mineralization of organic remains. — The changes which 

 fossil organic bodies have undergone since they were first imbed- 

 ded in rocks, throw much light on the consolidation of strata. 

 Fossil shells in some modern deposits have been scarcely altered 

 in the course of centuries, having simply lost a part of their 

 animal matter. But in other cases the shell has disappeared, and 

 left an impression only of its exterior, or a cast of its interior 

 form, or, thirdly, a cast of the shell itself, the original matter of 

 which has been removed. These different forms of fossilization 

 may easily be understood if we examine the mud recently thrown 

 out from a pond or canal in which there are shells. If the mud 

 be argillaceous, it acquires consistency on drying, and on break- 

 ing open a portion of-it we find that each shell has left impres- 

 sions of its external form. If we then remove the shell itself, 

 we find within a solid nucleus of clay, having the form of the 

 interior of the shell. This form is often very different from that 

 of the. outer shell. Thus a cast such as a. Fig. 54., commonly 

 called a fossil screw, would never be suspected by an inexperi- 



enced conchologist to be the internal shape of the fossil univalve, 

 ft. Fig. 54. Nor should we have imagined at first sight that the 

 shell a and the cast h, Fig. 55, were different parts of the same 

 fossil. The reader will observe, in the last-mentioned figure 

 (&, Fig. 55.), that an empty space shaded dark, which the shell 

 itself once occupied, now intervenes between the enveloping 

 stone and the cast of the smooth interior of the whorls. In such 



Fig. 54. 



Fig. 55. 



Phasianella Heddingtonensis, 

 and cast of the same. Coral Rag. 



Trochus Anglicus, and 

 cast. Lias. 



