THE STUDY OF CHEMICAL GEOLOGY. 
253 
the reactions between compounds contained in the ocean, in- 
fluenced by peculiar circumstances. 
The origin and mode of formation of the various rock masses 
and deposits which compose the crust of the earth, is a subject 
for investigation which cannot but yield results of great im- 
portance in the advancement of geological enquiry. 
All rocks which are as yet known may be roughly arranged 
under two heads — eruptive and sedimentary ; and both of these 
classes of rocks can be subdivided respectively, into normal and 
metamorphic, i.e. those which still are found comparatively* 
unchanged, and those which have suffered metamorphism or 
alteration, brought about by either mechanical or chemical force, 
or by both combined. 
The sedimentary strata, when comparatively unaltered, show 
themselves as tuffs, ashes, breccias, conglomerates, grits, sand- 
stones, shales, clays, marls, limestones, &c., and have been all 
formed either by the direct destruction of eruptive rocks or of 
previous sedimentary beds which, in their turn, had so originated; 
even the lime which organic life has eliminated from the ocean 
to form the limestones, came, if not altogether, at least in 
greater part, from the same source. This has been the case even 
from the very oldest period, oi*, in other words, from the epoch 
of the consolidation of our sphere, since the original crust of 
the globe must be regarded as representing what may be termed 
the first eruptive or massive silicated rocks. 
The study of the chemistry of the eruptive rocks becomes, 
therefore, a subject of special interest and importance, not only 
as tending to elucidate their origin and formation, but also as 
bearing on the nature of the sedimentary strata which, as before 
mentioned, have, directly or indirectly, been formed from their 
ruins. 
In Great Britain, it must be acknowledged, there is at this 
present time, little or no information on this subject in print ; 
and of the few chemical analyses of rocks which have been pub- 
lished, it is to be feared that many of them have been made on 
specimens which have not been selected with care, so as to re- 
present the actual rock mass in question. 
If the skill, time, and expense requisite in order to make 
even a single accurate analysis of a rock, is to be devoted to so 
doing, surely the first thing to be attended to is, that these 
should not be thrown away in vain; strange to say, a large 
proportion of all the published analyses of rocks, so far from 
being of use, are, on the contrary, worse than useless .to the 
geologist, as they mislead him by providing him with results 
* Everything in Nature appears, faster or slower, to become more or less 
altered. 
VOL. VII. — NO. XXYIII. T 
