50 



THE GEOLOGIST. 



The division into species is, in the 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 knowiedge 

 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 wlien we attempt to group these into a 

 systematic classification, and the leadiug mineralogists differ 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 ; 

 w^hile 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 elements, 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 J/a«?^c/^ q/ 6^eo/o(7// 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-foil }ths 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, has some important i*emarks on the classification of minerals. (See 

 chap. iv. p. 99.) He does not adopt the chemical ai-raugement. 



