ON CONGLOMERAtED ROCKS. 213 



In the sandstone, does not affect the plausibili- 

 ty of the opinion here stated. Sandstone some- 

 times occurs in globular and concentric lamellar 

 concretions, and in cotemporaneous veins, in 

 sandstone and trap rocks ; also in kidneys, in 

 slate- clay, amygdaloid,-and other rocks, appear- 

 ances expressive of the crystalline nature of the 

 sandstone, because they occur only in granite, 

 greenstone, and other crystalline rocks. If, then, 

 the opinion just stated in a very general way, shall 

 prove correct, it v/ill afford us an easy explanation 

 of all the varieties of curvature, hardness, and po- 

 sition, observed in sandstone strata ; and the al- 

 ternation of sandstone with limestone, trap, and 

 other rocks, will no longer be difficult of explana- 

 tion, 



Trap~tuj}\ the last conglomerated rock we enu- 

 metated and described, is generally supposed to 

 be entirely a mechanical deposite ; but I am in- 

 clined to view it more as a chemical than a me- 

 chanical formation. This opinion, I stated in my 

 account of the Pentland Hills ; and the following 

 are some of the facts that still induce me to con- 

 sider it as the most plausible explanation of the 

 mode of formation of that rock. 



1. The masses of basalt, amygdaloid, felspar, 

 and of other trap rocks contained in the tuff, are 

 to be observed gradually passing into the trap basis 

 m which they are contained. 



04 



