50 
THE GEOLOGIST. 
The division into species is, in tlio case of most minerals, natural and 
obvious ; but here, as in the animate kingdoms, the difficulty of 
drawing distinct lines between species increases as our knowledge 
extends. Indeed it must be admitted that the line of demarcation 
between many allied species is often drawn more from empirical and 
practical considerations than upon any recognisable scientific prin- 
ciple. But if we meet with difficulties in the establishment of species, 
there is yet greater confusion when we attempt to group these into a 
systematic classification, and the leading mineralogists diflfer com- 
pletely as to the principles upon •which it should be carried out. 
Some insist that they should be grouped solely according to their 
external forms ; some, according to their chemical composition alone ; 
■while others prefer an intermediate principle compounded of both. 
The chemical classification is that adopted in our standard English 
Mineralogy,* and is the one which we shall follow here, as bearing 
more nearly upon that higher portion of our subject, the investigation 
of the chemical genesis of rocks. 
X. In dealing with the chemical character of minerals and rocks, 
I must assume an elementary knowledge of chemistry on the part of 
my readers. I take for granted that they are aware that all matter, 
cognisable to us, consists of some one or more of the sixty-two 
elementary bodies, or eltments, each possessed of certain distinctive 
physical properties, and combining together according to certain 
laws; and that these compounds are distinguished by certain names 
■which indicate either their physical properties or their combining 
proportions. Those who have not this knowledge will find it neces- 
sary to acquire it, which can be done from any elementary treatise on 
chemistry. Mr. Jukes' recently published Manual of Geology contains 
an introductory chapter on chemical mineralogy, by Dr. W. K. 
Sullivan, to which every student would do well to refer. 
These sixty and odd elements are distributed with the widest 
inequality. One, oxygen, occurs so abundantly as to make up at least 
three-fourths of the terraqueous globe, and some others occur so fre- 
quently, and in such large quantities, as to form a notable proportion 
* Phillips's, before referred to. Professor Nicol, in liis Manual of Mineralogy, 
just quoted, lias some imi)ortant remarks on the classification of minei'als. (See 
chap. iv. p. 99.) lie does not adopt the chemical anaugement. 
