THE CLASSIFICATION OF IGNEOUS ROCKS. 481 



refinement. These, and subdivisions of them, while suf- 

 ficing for rough purposes, cannot, however, adequately 

 represent the complex conditions through which many 

 igneous rocks have passed during what we may describe 

 as a varied life-history. The record preserved in the rocks 

 themselves often gives evidence of successive stages of 

 consolidation under different conditions, of minerals crys- 

 tallised out at one stage and partially or wholly resorbed 

 at another, and of other changes of circumstances, either 

 gradual or abrupt. 



The assumption that the total chemical composition of 

 a partially fluid mass has remained constant through such 

 vicissitudes as are thus indicated, becomes on reflection a 

 very improbable one. Nor can we set aside the numerous 

 cases adduced in late years to prove that chemical, as well 

 as mineralogical differentiation has been brought about 

 under suitable conditions in originally homogeneous rock- 

 magmas. As regards the nature of these conditions, and 

 the mode of operation of the differentiating processes, almost 

 everything is still to be learnt ; but the facts seem to justify 

 the belief that the bulk-analysis of an igneous rock, as well 

 as its mineralogical and structural characters, may often be 

 dependent in greater or less degree upon the conditions 

 governing its consolidation, the circumstances attending 

 its eruption, and other facts in its history. Light will be 

 thrown upon the subject both by the study of individual 

 rocks and by the comparison of groups of associated rocks 

 to which we are warranted in attributing a community of 

 origin. 



The idea of relationship or "consanguinity" among 

 igneous rocks, pointing to some kind of common origin, 

 brings us to contemplate the possibility of a classification 

 based on genetic considerations. This, as already re- 

 marked, can at present be regarded as only a distant goal, 

 and to follow up the subject would lead us into untrodden 

 paths. We shall content ourselves with noticing the work 

 of Brogger (35), who has made a bold attempt to apply the 

 principles of differentiation to a systematic arrangement of 

 the numerous peculiar rock-types of the Christiania basin. 



