IGNEOUS ROCK-SERIES AND MIXED ROCKS 399. 



minerals as cordierite, sillimanite, corundum, spinel, idocrase, and 

 others, which are either quite foreign to normal igneous rocks or 

 at least foreign to rocks of the general type of those concerned. 

 A mixture of two igneous rocks will in general show less obvious 

 peculiarity, but it may still be expected to betray itself in the 

 occurrence of unusual minerals, unusual mineral-associations, or 

 unusual relative proportions of the constituent minerals. That 

 it often does so betray itself in a fashion quite unmistakable, is 

 proved by numerous examples of undoubted mixed rocks which 

 have come under the notice of the present writer. 



The question here broached has a very direct application to a 

 subject now much in the minds of penologists, viz., the 

 endeavor to arrive at some natural (as opposed to a merely Lin- 

 naean) classification of igneous rocks. Such a classification must 

 be based, confessedly or implicitly, upon fundamental genetic 

 considerations, and primarily upon the mode of operation of the 

 processes of differentiation in rock-magmas. Rocks resulting 

 from admixture must therefore be excluded from the main 

 scheme and relegated to an appendix. Any discussion which 

 tends to the recognition of this principle and to the establish- 

 ment of some criterion of distinction will forward the object by- 

 disembarrassing the problem of a disturbing element. 



Alfred Harker. 



