66 JOURNAL OF GEOLOGY—SUPPLEMENT 



alkaline types entirely independent of absorption of foreign material 

 in the great majority of cases and would assign the significant role 

 to juvenile gaseous substances, principally water, concentrated in 

 residual liquors in a natural way. 



IS BASALTIC MAGMA THE PARENTAL MAGMA OF ALL IGNEOUS ROCKS ? 



GENERAL CONSIDERATIONS 



In the foregoing pages an attempt has been made to give a 

 rather full discussion of differentiation in igneous rocks and of the 

 principles underlying it. A beginning was made with the dis- 

 cussion of the common association, in sill-like bodies where both 

 top and bottom are exposed and all the relations clear,^ of diabase 

 with biotite granite and often several intermediate types. A 

 specially notable case is that of the Purcell sills described by Daly 

 and later by Schofield in greater detail. It was shown that the 

 relations found are strictly those to be expected if differentiation 

 takes place through crystallization under the influence of gravity. 

 The relation of the biotite granites and related biotite-quartz types 

 to the alkaline rocks was then discussed and it was shown that the 

 same principles of derivation are applicable. 



While no implication is intended in the foregoing that basaltic 

 magma is always the parent magma of the igneous series given, and 

 while the outline may equally well be regarded as a contribution to 

 the study of the differentiation of dioritic magma or of granitic or 

 other magmas, nevertheless the discussion does emphasize the fact 

 that a great variety of igneous rocks, possibly all important types, 

 could be derived from basaltic magma by differentiation under the 

 appropriate conditions. Since the idea that basaltic magma is the 

 parent magma of all igneous rocks has already found favor with 

 some petrologists, it becomes advisable to discuss the matter in 

 the present connection. 



Of the visible igneous rocks of the globe the volume of salic 

 types is many times that of femic types. ^ In view of this fact it 

 may seem a bold step to express the opinion that basaltic magma 



^ A method of attack the importance of which Daly has long emphasized. 

 ^ See Daly, Igneous Rocks and Their Origin, p. 52. 



