CHEMICAL AND PHYSICAL CHARACTERISTICS OV ROCKS. 419 



them as we now find them to exist, but to endeavour to deduce, from a 

 careful comparative study of their chemical constituents and chemical 

 changes, the laws of their formation, the circumstances of their origin 

 and subsequent changes, and generally to investigate their genetic 

 history." K'or is this division of the study of rocks — which has been 

 called chemical geology — a subject of minor or even secondary import- 

 ance. Upon the result of these investigations depends the solution of 

 many geological principles of the very first importance which palceon- 

 tology is powerless to unravel. The great question of the metamorphic 

 origin of the crystalline schistose rocks, and even of granite itself, result- 

 ing from processes similar, both in kind and degree, to those now in 

 action beneath our surface, can only be proved or disproved by patient 

 investigations in this branch of science. And upon this, again, depends 

 the final decision as to which of the two contending doctrines of geology 

 shall prevail — that of Uniformity or that of Progression : whether, in this 

 world of ours, we can trace back the evidence of the existence of another 

 order of things — a state of incandescence gradually cooling down, 

 followed by alternate periods of tranquillity and of great convulsion ; or 

 whether all evidences of a beginning are beyond our view, the present 

 order of things having rolled uniformly on during ages vast 

 beyond our imagination — marked certainly by periods of disturb- 

 ance and of repose, succeeding each other in different parts of the globe, 

 but uniform on the whole. With such objects and investigations for 

 ultimate results none can consider this branch of geology unworthy of 

 attention ; at least no one who wishes to keep himself up with the 

 progress of geological science should fail to acquire a sufficient know- 

 ledge to enable him to follow the labours of others, if even he 

 feels no desire ultimately to follow it out practically in the field or the 

 laboratory. 



YII. Although no work has, of late years, been published in the Eng- 

 lish language on the subject of rocks, many important works have 

 appeared abroad, particularly in Germany. Within the last three 

 years, there has been published in that country Professor Cotta's 

 OesteinUhre,^ Dr. Senft's ClassificoMon and Description of Roclis,] 



* Cotta. — Die Gesieinskhrc. Freiberg, 1855, pp. 255. 



t Scnft. — Classification und Deschreihing der Felsarten. Breslan, 1857, pp. 442. 

 An abstract of the leading chapters of tliis work may he found in the *' Quarterly 

 Journal" of the Geological Society, Vol. XlV., part 11, miscell. p. 1., &c. 



