DIFFERENTIATION IN MAGMA BASINS 557 



(p. 59). My belief has been, and is, that the differentiation of 

 these two great classes of magmas from the common stock and the 

 separation of them — in general in a horizontal sense — constitute the 

 first and most important steps in the evolution of igneous rocks. 

 Why the chemical differentiation should so consistently follow these 

 lines has been a difficult problem, and it is the more gratifying to be 

 offered at least a partial answer to the question. Stated broadly, 

 Bowen's ideal scheme of differentiation leads first to a series of 

 calcic rock types and subsequently, if continued, to an alkaline 

 series. There are qualifications of this rough statement which I 

 do not go into here; but in general it appears that, if a separation 

 can be brought about at a certain well-defined stage of the progres- 

 sive differentiation, it will be a separation between calcic and alka- 

 line. This separation, I hold, has actually been effected on a grand 

 scale, and I have sought the immediate cause of it in the action of 

 crustal stresses squeezing out the residual fluid. 



A discussion of this suggestion from the chemical point of view 

 would be instructive, but here Bowen disappoints expectation. 

 He dwells on particular cases in which separation has not taken 

 place at the stage specified, but at a somewhat earlier stage; and 

 he throws doubt upon the existence of any general regional dis- 

 tribution of alkaline and calcic rocks, such as Iddings demon- 

 strated long ago. The fact that, among the younger rocks of North 

 America, alkaline types characterize the Atlantic slope and calcic 

 the Pacific, he would explain by supposing that erosion has exposed 

 deeper levels on the western side of the Rocky Mountains than on 

 the eastern. He forgets that the contrast of petrographical facies 

 holds good for the lavas as well as for the intrusive rocks. More- 

 over, the fact that lava flows still cover vast areas on the western 

 side, while on the eastern they have mostly been removed, makes it 

 difficult to accept his statement about the relative amounts of 

 erosion. 



As regards the association of calcic rocks with regions subjected 

 to powerful lateral thrust, nothing would be gained by traversing 

 old ground again, but to Bowen or any other unbeliever I will offer 

 just one consideration. If we examine those crystalline schists 

 which are admittedly of igneous origin, together with foliated 



