S13 



cases, present similar appearances, but which, not 

 being of very great importance in the present view, I 

 shall not take into further consideration. 



2dly. I shall proceed to examine the quartzose 

 rock, compact mica slate, clay slate, primitive sili- 

 cious slate, horn stone slate or the palaiopetre of 

 Saussure, &c. 



These substances, it is well known, when compactly 

 or perfectly formed, have each an alluminous or sili- 

 cious cement, which gives to the substance such a 

 degree of elasticity or firmness that, if they do absorb 

 a small quantity of water (and small indeed it must 

 be) they are capable of resisting the operations of cold 

 and frost. Hence, the extent of disintegration with 

 rocks of this description is very limited. This con- 

 clusion however is drawn from the appearances which 

 they generally present to view. That is, of being 

 angular or sharp pointed, and extremely craggy. 



Where this is the case, it is highly improbable that 

 they can have suffered any material loss by disintegra- 

 tion : for, as before mentioned, it is next to impossible 

 that this can take place, no matter what may be the 

 agent employed, for the purpose, without the points 

 and angles being reduced and blunted, thus gradually 

 progressing to a rounded form. 



3dly. I shall consider the green-stone, (or diabase 

 of Brogniart) the porphyritic rocks, some of the 

 amygdaloids, &c. 



From all the opportunities which I have had of 

 examining the^e tucks in place, and from the observa- 



