498 OVERLYING ROCKS. 



daloidal in another. Where Nature is indefinite, 

 the student must not be disappointed if he can- 

 not frame or find a definite artificial arrangement. 

 It must be remarked of the preceding rocks, 

 and chiefly of varieties C, D, E, that they often 

 lose their natural characters, even to a great depth 

 from the surface, becoming weathered ; if that 

 term can be used to designate a change where 

 the weather can scarcely obtain access. They 

 are not, in these cases, disintegrated, but retain 

 their solidity, together with the appearance of 

 natural rocks. In such circumstances, they ac- 

 quire an arenaceous aspect, or become carious ; 

 undergoing, at the same time, changes of colour ; 

 and in this way the dark indurated claystones or 

 clinkstones, will put on the appearance of com- 

 mon claystone or even that of the indurated clay, 

 var. B. 



G. Hornblende compacted into a solid mass, and 

 apparently consisting of minute crystalline particles. 

 Hence the fracture is more or less coarse grained, and 

 is, further, uneven, splintery, or conchoidal. It varies 

 in lustre, being sometimes considerable glistening. 

 This is one of the varieties of basalt: it is imagined to 

 be the only basalt by some authors. 



