for the Study of Megadiastrophism. 



411 



particular case presents an a posteriori problem of its 

 own. It is wholly unembarrassed by any requirements 

 that its factors shall sum up into a speculative primitive 

 magma. ' ' 



From this scheme of magma derivation two broad con- 

 clusions may be drawn : First, the tongue melts reaching 

 the surface should be, at least, within continental areas, 

 generally acidic in composition, since they are supposedly 

 derived from the selective liquefaction of the more fusible 

 constituents of an earth whose composition is similar to 

 the visible rocks and this composition is that of common 

 granite; second, the acidity should decrease towards 

 basicity with time since the process implies the lique- 

 faction of less and less fusible constituents as interior 

 condensation progresses. 



On the other hand the outstanding fact of the occur- 

 rence of the igneous rocks is the striking uniformity in 

 composition of the great extrusive flows, this similarity 

 extending in time from the most ancient extrusive masses 

 of Archean age down to the present, and in location world 

 wide. Diversification is absent. And the composition of 

 these extrusives is predominantly basaltic. Acidic extru- 

 sive s are conspicuous by their almost complete absence. 

 Derivation of these great and persistent outpourings of 

 basaltic material from an essentially acidic earth is not 

 only difficult to explain by the process of selective lique- 

 faction but, on the other hand, if they are considered as 

 differentiates of a more acidic magma, then, as Daly has 

 pointed out : 41 "The acid pole of such a hypothetical split- 

 ting ought to be on a similarly large scale." Nor can the 

 acid intrusive masses fill the need, in this case, of the 

 acidic derivative, for then a further explanation would 

 be necessary to account for the predominant intrusive 

 form of the acid and the predominant extrusive form of 

 the basic poles. Furthermore, the fact of discordancy 

 of the batholitliic contacts together with the fact that the 

 structural delineaments of roof and wall rocks are merely 

 truncated at the contact surfaces rather than altered 

 implies that large masses of these rocks have been 

 engulfed and have added their high acidity to a previously 

 more basic magma. 



41 E. A. Daly: Op. eit. ; p. 164. 



