PRIMARY MAGMA 633 



It may be argued that the intrusion of a partly differentiated magma 

 separating it from its original basaltic products would be accompanied 

 by a change of pressure and water content, et cetera, so that the course 

 of differentiation might be changed. Intrusion, however, would hardly 

 be expected to normally increase the water content, and some of the 

 series plotted grade toward alkaline types in such a way as to indicate a 

 high content of miner alizers. As a whole, therefore, the diagrams are 

 less indicative than other lines of evidence that basalt was "the primary 

 magma. w 



DEGREE OF UNIFORMITY OF PRIMARY MAGMA 3C 



There are many reasons for the prevalent idea that primary magma is 

 homogeneous. Under the theory of a molten earth, or even a molten sur-, 

 face layer, there would be abundant opportunity for general stirring and 

 mixing to promote uniformity. Within any large body of magma, con- 

 vection and diffusion tend toward homogeneity. It is now popular to 

 emphasize the similarity of magmas and solutions, and one of the text- 

 book characteristics of solution is the tendency to become homogeneous. 



The field evidences are also strong. 37 Basalt penetrates the upper 

 earth without associated acid rocks in so many cases and with such uni- 

 formity of composition that it can not be considered as freshly differ- 

 entiated from magma of intermediate composition. The differentiation 

 of basalts seems to be related to the size of the mass, granites being a 

 sign of large masses; and the bodies of acid rocks, the only rocks of 

 greater known volume than basalts, are very large and their lower phases 

 concealed. Finally, and perhaps most significant of all, is the observa- 

 tion that basalt is the most widespread in time and space of all the lavas 

 that seem to have come rapidly from the regions of magma generation 

 to their solidification, and the chief rock in all extensive lava fields. 



Arguments against uniformly basaltic magma (cited above) are 

 found in the scarcity of intermediate rocks; the occurrence of some in- 

 termediate rocks as cores related to differentiated flows ; the difference in 

 pre-Cambrian and post-Cambrian magmas; and some new calculations 

 based on isostasy. 



Whatever the basis for the conclusion, both the authors, arguing for 

 basaltic magma as "primary," refer to its "uniformity." 



86 The uniformity of composition of a rock family depends largely on the strictness of 

 definition of the family. Basalt is very variable if it includes rocks with and without 

 feldspathoids and with and without olivine. As an example of a more strict definition, 

 granodiorites are defined by Lindgren with relatively narrow limits. No one, however, 

 would suggest that granodiorite must be a primary magma because of its uniformity. 



87 R. A. Daly : Op. cit, pp. 164-165. 



