110 



In other cases, two or more distinct rocks unite 

 to form one series; being repeated in an order 

 which may be either regular or otherwise. Thus 

 gneiss is associated with hornblende schist. 



Sometimes, a small proportion of one rock 

 occurs in a series principally consisting of another. 

 The term subordinate has been applied to such a 

 rock ; occasionally with no small inconvenience. 

 Shale alternating with sandstone, is an example of 

 this disposition. 



It is a prevailing rule in nature for a given stra- 

 tum to consist of one rock. It also most com- 

 monly consists of one modification of that rock ; 

 but this rule is far from universal. Thus, the 

 upper part of a stratum may consist of a fine sand- 

 stone, and the lower of a conglomerate ; or the 

 reverse. This change may also take place ac- 

 cording to the plane of the stratum, or laterally. 

 Strata frequently contain foreign organic bo- 

 dies imbedded ; sometimes confusedly, at others, 

 with more or less of conformity to the plane of the 

 stratification. 



There are cases jn which it may be difficult to 



