316 SIR JAMES HALL on the Consolidation of the Strata. 



der which are comprehended all substances of this kind, includ- 

 ing not only Whinstone and Basalt, but also Porphyry, Granite, 

 and Sienite of every description. 



The solid mass of our globe, then, in so far as it is naturally 

 exposed to our view, or has been penetrated by the labours of 

 the miner, would appear, (with the exception of some streams 

 which have flowed from Vesuvius, Lipari, and other volcanoes, 

 in which the rock possesses a glassy structure), to be compre- 

 hended under these two classes, Aggregates and Crystallites. 



The whole of these rocks, of both classes, furnish, at every 

 turn, proofs of their having undergone revolutions of the utmost 

 magnitude ; and much ingenuity has been exerted, in endeavour- 

 ing to trace these changes to some consistent and rational sys- 

 tem. But of all the active powers of nature, one only has occur- 

 red to me as capable of affording a solution, in any degree satis- 

 factory of the phenomena, I mean the power of internal heat, 

 which, in all ages, and in various countries, has made its appear- 

 ance at the surface of the earth, not unfrequently from under 

 the ocean, and which still, in our own days, gives occasional 

 proofs of its unabated activity. 



To ascertain the reality and sufficiency of this agent, and to 

 trace the volcanic fire to its source, with tolerable probability, is, 

 doubtless, an object of great interest and curiosity ; but it has al- 

 ways appeared to me, that the progress of geology was retarded 

 by a premature anxiety to enter into such investigations. 



Taking it for granted, however, as, indeed, no one can dis- 

 pute, that there frequently do arise violent exertions of heat from 

 under the bed of our ocean, Dr HUTTON held that this might 

 furnish a rational and sufficient theory of the earth, without en- 

 tering into any inquiry as to the origin of that heat ; and admit- 

 ting that there are many geological facts which cannot be ac- 

 counted for by such a fire as that of Vesuvius, now acting at the 

 surface, in free communication with the air, he contended that 



