THE CLASSIFICATION OF IGNEOUS ROCKS. 



THERE is perhaps no branch of study in which the 

 accumulation of material has so far outrun the ad- 

 vance of systematic science as in Petrology, and to the zeal 

 thus shown on the purely descriptive side of the subject 

 may be attributed in some measure the confusion so 

 apparent in the classification and nomenclature of the 

 igneous rocks. There are, however, in the nature of the 

 subject itself, considerations which seem to make classification 

 a matter of peculiar difficulty. Petrology knows no species 

 or genera. The laws of precision, chemical and crystallo- 

 graphic, which give individuality to minerals, and lead to 

 the grouping together of allied minerals, afford us no help 

 in dealing with complex mineral aggregates such as igneous 

 rocks. Neighbouring rock-types, and even widely diverse 

 types, are linked together by an indefinite series of inter- 

 mediate varieties, and are found in the field to o-raduate 

 into one another by insensible transitions. Of any such 

 fundamental principle as that of descent, which defines 

 species and .underlies classification in the organic world, 

 petrologists have obtained as yet only a dim glimpse. It 

 is perhaps in this direction that the solution of the problem 

 is to be sought, but a clear conception of all the physical 

 and chemical principles involved in the genesis of rocks 

 from molten silicate-magmas is a necessary preliminary 

 which is at present far from being realised. Meanwhile, 

 it may be of interest to recall some of the steps by which 

 the present position of the question has been reached. 



To trace completely the growth of opinion with regard 

 to the classification of rocks would be to write a history of 

 Petrology. Moreover, the earlier writings on the subject, 

 though full of interest historically, scarcely touch the science 

 as now understood, and therefore need not engage our 

 attention. Instead of following a chronological sequence 

 it will be more instructive to consider briefly the several 

 points of view of those who have approached the subject 



