LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 73 



which initiate the volcanic phase of the igneous activity of the 

 region in question is more likely to represent the original magma. 

 In the Christiania region of Norway, for example, the average 

 composition of the rocks is somewhere near nordmarkite. If it is 

 considered that all the rocks were derived from nordmarkitic 

 magma, it is necessary, in order to explain the sequence of types, to 

 imagine a differentiation while the magma was still Kquid, as 

 Brogger does, and a collection of the heavier basic differentiates 

 toward the top. This conception Harker has rightly described, 

 in a different connection, as "wholly chimerical,"^ but one might 

 refer in a similar manner to the deep-seated magma basin, stratified 

 according to density, as it might be, but from which, in the usual 

 case, "the earlier intruded magmas are drawn from the lower 

 levels."^ On the other hand, if it is assumed that the original 

 magma was the gabbro which forms the earliest extrusives, the 

 rocks of the Christiania region seem explicable as the result of 

 gravitative differentiation increasing in importance with the size 

 of the bodies and emphasizing the alkaline and saHc types in the 

 exposed portions of the larger bodies. In the very closely related 

 JuHanehaab region Ussing has called special attention to "the 

 contrast between the mode of occurrence of the dense, gabbroid 

 rocks and the specifically lighter, more acid rocks," the former 

 as dykes of small volume and the latter as batholiths, or rather 

 the upper part of batholiths.^ 



THE EARLIER STAGES OF IGNEOUS ROCK EVOLUTION NOT REVEALED 



The facts which have been pointed out in favor of considering 

 basaltic magma the parent of all igneous rocks do not, of course, 

 definitely prove the case, yet they appear to estabhsh a strong pre- 

 sumption in its favor. The geologic record makes it necessary 

 to imagine a source of basaltic magma beneath all parts of the 

 surface of the earth throughout recorded time, and a great many 

 facts indicate the derivation of all other magmas from the basaltic. 

 What the antecedent stages which permitted the formation of 

 this source of basaltic magma may have been is not revealed in the 

 record. Consideration of these early stages would lead one into 



^ Op. ciL, p. 317. 2 Ibid., p. 330 (italics are mine). ^ Qp. cit., p. 308. 



