WARREN. — ALKALI-GRANITES AND PORPHYRIES. 325 



engulfed in the magma beneath. Under the thick cover of porphjTies 

 acting as an effective insulator, as well as along the deeper original 

 contacts, the remaining magma cooled with sufficient slowness to 

 permit of the development of a medium coarse, even grained rock, 

 the granite. 



Locally, against deep projections of slate, as well as to a small 

 degree under its own porphyry cover, the magma diiferentiated with 

 the formation of a relatively basic phase, the rhombenporphyry. 

 Some part of this basic differentiate still remains in place against the 

 slates, but considerable portions of it were broken up, and together 

 with fragments of the other contact phases, were scattered by sinking, 

 and movements in the magma. ]\Iany of these were e\'entually 

 frozen in the magma, particularly near their original places of forma- 

 tion, and now form the cognate xenoliths (knots) so abundant in the 

 granite. 



Long continued erosion has removed nearly all of the original 

 slates etc. invaded by the magma, together with some portion of the 

 first formed marginal cover of igneous rocks. Faulting and elevation 

 of the mass, particularly along its northern edge, has brought up into 

 the plane of erosion a portion of the original deep seated slate-granite 

 contacts. 



Owing probably to the small capacity possessed by the magma for 

 differentiation, combined with the rapid refrigeration of a great part 

 of the magma, no complementary dike phenomena or other intrusions 

 of complementary nature were developed, and are, in striking contrast 

 to many other batholithic intrusions of granitic rocks, al)sent from 

 this area. 



In the case of the rhombenporphyry and the cognate xenoliths 

 derived from it, it is held that their crystallization followed differen- 

 tiation, and that the process of differentiation was one of diffusion of 

 compounds, which were normally the first to crystallize from tlie 

 magma, toward the cooling surface. It is held that while a part of 

 the sharp contacts between this basic phase and the granite-porpliyry 

 and granite are due to mechanical causes (breaking and movements 

 in the magma) there was originally a practical discontinuity — sharp 

 or very sudden change — between the rhombenporphyr;v' and the 

 main magma. 



Chemically the magma as a whole is characterized by liigh silica, 

 high alkalies, relatively high iron and by very low lime and magnesia. 

 The alkalies are about equal, the potash slightly predominating in 

 amount, but molecularly less important than the soda. Chief ac- 

 cessory constituents are zirconium, titanium and fluorine. 



