CHEMICAL AND PHYSICAL CHARACTERISTICS OF 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." Nor 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 I'esult of these investigations depends the solution of 
many geological principles of the very first importance which palteon- 
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. 
VII. 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 
Gesteinlehrci'^ Dr. Senft's Classification and Description of Rochs,] 
* Cotta. — Die Gealcinslchre. Freiberg, I860, pp. 255. 
t Scnft. — Classification iind Dcschrcihtinr) der Fdsarkn. Broslau, 1857, pp. 442. 
An abstract of the leading chapters of this work may be found in tlic " Quarterly 
Journal" of the Geological Society, Vol. XIV., part II, misccll. p. 1., &c. 
