202 



STRUCTURAL GEOLOGY. 



best illustrated by the case of shells. The following 

 figure is a diagram showing four different cases, all of 

 which are very common. In the figure the horizontal 



FIG. 118. Section of strata containing fossils. 



lines represent the stony matrix in which the shell is 

 formed, or mud in which the shell was originally buried, 

 and the vertical lines represent the subsequent filling 

 with finer material. 



Explanation. In case a, the living or recently dead 

 shell was buried in mud, and afterward the whole organ- 

 ism was dissolved and 'removed, leaving only the hollow 

 mold where it lay. In case 5, we have the same, only the 

 mold has been subsequently filled and a cast made by the 

 deposit of silica or carbonate of lime from solution. If 

 the rock be broken, the cast will often drop out of the 

 mold. In c, the dead, empty shell was buried in mud and 

 filled with the same, and afterward the shell was removed 



FIG. 119. a, Natural form ; &, FIG. 120. Trigonia longa, showing cast (a) of the 

 cast of interior and mold of exterior and (6) of the interior of the shell 



exterior. 



