LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 71 



ranes which has led to the exposure in those areas, and almost solely 

 there, of large bodies of anorthosite which should under the present 

 conception of its origin be a moderately deep-seated differentiate. 



Another fact which has been advanced in favor of the idea that 

 basaltic magma is the parent of all igneous rocks is its continual 

 recurrence in terranes of all ages in the form of fissure eruptions.^ 

 It occurs likewise as dykes filling fissures related to the primary 

 structures of all terranes, which dykes are indeed often the feeders 

 of fissure eruptions. These occurrences indicate its sudden arrival 

 from great depth without opportunity for differentiation en route. 

 All other types of magma when occurring in dyke form tend rather 

 to center about areas of bathoKthic intrusion, the seats of differ- 

 entiation. 



It may, perhaps, be considered that it is the great fluidity of 

 basaltic magma which permits it to pour out freely in the manner 

 referred to. If this were true it would seem that basaltic magma 

 would be the commonest material of fissure eruptions, andesitic 

 magma somewhat less common, dacitic magma less common still, 

 and so forth. Apparently, however, the material of fissure erup- 

 tions is too commonly basaltic to agree with this supposition. It 

 is this fact which suggests that it is the only magma which arrives 

 at the surface undifferentiated and that other magmas are formed 

 from it by differentiation when it collects in bathoHthic masses. 



Some further support for the conception that basaltic magma 

 is the parent magma of all igneous rocks is to be found in the 

 mineral constitution of the rocks themselves. Granitic or any 

 closely related magma could not give a differentiate of basaltic 

 composition by the collection of its heavier minerals. Only 

 dioritic magma might reasonably be expected to do so at any time, 

 but it would be remarkable if it gave at nearly all times basaltic 

 magma of approximately uniform composition. One would expect 

 rather a marked variation, according to the opportunity for the 

 sorting of crystals of different densities, in all rocks more "basic" 

 than the parental magma. This is true only of all rocks more basic 

 than basalt. Pyroxenites, peridotites, and here we should include 

 anorthosites also, are notably variable in the proportions of their 



^ Daly, Igneous Rocks and Their Origin, p. 458. 



