EQUIVALENT. 



157 



seam has only been dislocated and moved to different levels, 

 by means of 'faults, as figs. 97 and 98. 



PIG. 97. 



EQUIVALENT. Where one bed supplies the place of 

 another, which in that situation is wanting, such stratum is 

 called the equivalent of the missing deposit. "When a bed 

 suddenly terminates, and its place is supplied by one of 

 different character, the latter is also called the equivalent of 

 the former. 



FIG. 98. 



There are some other varieties of texture, as well as 

 different phenomena in rocks, which may be here mentioned. 



PISOLITES. The pisolitic structure in certain stones is 

 occasioned by isolated globules formed of concentric layers. 

 They are produced by water holding foreign substances in 

 solution, and possessing such a power of motion as to enable 

 them continually to transport with them the grains of sand 

 occurring in their course. Each of these grains becomes 

 covered on all sides with successive pellicles of the substance 

 deposited by the fluid, and it thus increases, assuming the 

 spheroidal form, until becoming too heavy for transport it 

 falls to the bottom of the liquid, where the whole are agglu- 



