METAMOEPHIC RCCKS. 245 



tion, and by bringing to the aid of inductive conclusions, 

 direct chemical experiments on the effects of different 

 degrees of fusion and pressure, and different rates of 

 cooling. When the study of chemical combinations is pur- 

 sued under the guidance of leading ideas, ( 258 ) a bright light 

 may be thrown on the wide field of geology, and on the 

 operations of the great laboratory of nature, in which sub- 

 terranean forces have formed and modified the terrestrial 

 strata. But to avoid being misled by apparent analogies to 

 entertain too narrow a view of the processes of nature, the 

 philosophical inquirer must ever keep in view the compli- 

 cated conditions, and the unknown intensity, of the forces 

 which, in the primitive world, modified the reciprocal action 

 of the several substances. It cannot, however, be doubted 

 that the elementary substances always obeyed the same laws 

 of affinity ; and I am fully persuaded, that where apparent 

 contradictions are met with, the chemist will generally suc- 

 ceed in explaining them, by ascending in thought to the 

 primary conditions of nature, which cannot be identically 

 reproduced in his experimental researches. 



Observations made with great care, and over considerable 

 tracts of country, shew that erupted rocks have acted in a 

 regular and systematic manner. In parts of the globe most 

 distant from each other, ( 259 ) granite, basalt, and diorite, are 

 seen to have exerted, even in the minutest details, a perfectly 

 similar metamorphic action on the argillaceous schists, the 

 compact limestone, and the grains of quartz in sandstone. 

 But whilst the same kind of erupted rock exercises almost 

 every where the same kind of action, the different rocka 

 belonging to this class, present, in this respect, very 

 different characters. The effects of intense heat are indeed 



