484 SCIENCE PROGRESS. 



graduated succession of types which may form parts of a 

 single body of eruptive rock, such as a laccolite. The 

 variations, chemical, mineralogical, and structural, in such 

 a suite are the result of differentiation in the laccolite itself ; 

 while the differences between the several members of a 

 series are due to " tiefmagmatische" differentiation, effected 

 in the parent magma-basin prior to the intrusion. 



In the case of such an assemblage of igneous rocks as 

 those of the Christiania district, with so strongly marked 

 consanguinity, it can scarcely be doubted that all have 

 originated, by differentiation of more than one order, from 

 a common stock. If similar rocks in other regions have had 

 a like origin, a complete knowledge of the mutual relations 

 of the rocks should lead to a natural classification founded 

 on fixed genetic principles. 



The condition here expressed seems to be a necessary 

 one. It is, of course, no objection to such a system as is 

 here contemplated that it brings under one family rocks 

 which differ widely in composition as well as in characters ; 

 but if one rock-type may figure under two or more families, 

 the system seems to be lacking in simplicity. B rugger has 

 remarked, for example, how bostonite and camptonite may 

 arise as complementary products of differentiation of either 

 a gabbro- or a nepheline-syenite-magma ; so that, in a 

 genetic grouping of general application, the camptonites of 

 Norway and of New England would fall into two widely 

 separated families. If the facts have been rightly inter- 

 preted, such anomalies must be inseparable from a general 

 classification founded on genetic relationships. In any 

 case, such work as Brogger's, involving minute comparison 

 of many associated rock-types with reference to their mode 

 of origin, cannot fail to throw light upon the problem of a 

 natural classification. 



In the foregoing pages no idea has been given of the 

 details of the various classificatory schemes advocated by 

 different petrologists. This can be obtained from the works 

 cited below. They are selected from a rather voluminous 

 literature to represent the different ways in which the sub- 

 ject has been approached, and in some measure the develop- 



