184 



It is unnecessary to describe the transitions of 

 granite into the porphyries, greenstones, and 

 basalts of the secondary unstratified rocks ; as 

 these are the consequences of affinity, not of con- 

 tact, and are fully detailed elsewhere. 



Among the stratified rocks, one of the most 

 prevailing associations is that of gneiss with horn- 

 blende schist. Between these two substances, 

 the gradation is so frequent, and often so perfect, 

 that numerous examples are perpetually occur- 

 ring, which may, with equal propriety, be re- 

 ferred to either, 



The association of gneiss with micaceous 

 schist is also very common ; and in this case, 

 when the felspar, or the hornblende, of the for- 

 mer, becomes very scanty, the rock, although, in 

 a rigid sense, still appertaining to gneiss, cannot, 

 without great care, be distinguished from mica- 

 ceous schist, 



In Scotland, at least, the association of gneiss 

 and quartz rock is more common than the pre- 

 ceding; and the gradation here, is not only fre 



