OVERLYING ROCKS. 511 



either in the form of mountain masses, or in 

 veins. 



The three first varieties, a, b, c, occur chiefly 

 among the latest secondary strata, but are occa- 

 sionally found traversing even granite, like the 

 simple rocks of which they are varieties, in com- 

 pany with var. d. This latter variety occurs 

 almost exclusively, or at least most commonly, 

 in situations where it may be supposed of far 

 more antient date than the former. 



Varieties a, b, and c, are occasionally laminar, 

 like the syenites already enumerated, and also co- 

 lumnar. The var. d, is sometimes also of a 

 laminar structure ; but I am uncertain if it has 

 yet been found disposed hi the columnar form. 



e. The base of basalt. Basaltic porphyry. 



This rock occurs, like the simple basalts, 

 among the latest strata, and under all the same 

 modifications. The crystals of felspar are very 

 frequently glassy ; and, as the base may consist 

 of any of the substances already mentioned as 

 known by the name of basalt, it might be sub- 

 divided into varieties, were such refinement ne- 

 cessary. 



