IGNEOUS ROCK-SERIES AND MIXED ROCKS 393- 



found among members of different series belonging to the same 

 suite of eruptions in one district. Most of those who have dis- 

 cussed the origin of igneous rocks have sought the cause of this- 

 diversity in various processes of diffusion, etc., commonly spoken 

 of as differentiation in rock-magmas ; it is no part of our present 

 object to discuss these processes. Some geologists, however,, 

 including Professor Sollas and Dr. Johnston-Lavis, have laid 

 stress on the possible origin of certain igneous rocks by admix- 

 ture, a process in some sense the reverse of differentiation, and 

 this question we shall consider more closely. 

 We may distinguish a priori three cases : 



1. Mixture of two fluid rock-magmas. 



2. Permeation or impregnation of a solid rock by a fluid magma 

 with consequent reactions between the two. 



3. Inclusion of solid rock-fragments (xenoliths of Sollas) in a 

 fluid magma and their partial or total dissolution and incor- 

 poration in the magma. 



In the first case the two rocks involved must be of the same 

 age and presumably from a common origin. In the second and 

 third cases this is not necessarily true, and the solid rock need 

 not even be an igneous one ; but, when we examine actual 

 instances which have been described, it seems probable that here 

 also admixture does not in fact take place on an important scale 

 except between igneous rocks of cognate origin. Lacroix, in 

 his exhaustive memoir on xenoliths, 1 distinguishes two catego- 

 ries, enclaves etiallogenes, which are not related in composition or 

 by origin to the enclosing rock [e. g., limestone fragments in 

 trachyte), and enclaves homceoghies, which do present more or 

 less resemblance in composition and origin to the rock in which 

 they are enclosed (e. g., olivine-nodules in basalt). Similarity 

 of mineralogical composition is, however, by no means a suffi- 

 cient test of community of origin among igneous rocks, and 

 instances may easily be cited (e.g., some cases of gabbro enclosed 

 in granite) which would be placed by Lacroix under the former 

 of his two heads, but in which there exists, despite differences of 



J Les enclaves des roches volcaniques, Macon, 1893. 



