COMPLEMENTARY ROCKS. 355 



which is nearly constant. The snni of the alkalies decreases with decrease 

 of silica and alumina. Magnesia and lime are fairly high. In the two 

 rocks of Square Butte the shonkinite is like that of Yogo Peak, but alumina 

 and alkalies are somewhat lower, and magnesia and lime higher. The 

 syenite of Square Butte, the complenientary rock of shonkinite, is high in 

 alumina and alkalies and very low in magnesia and lime, with very high 

 potash. In these two cases we find shonkinites as extreme forms of differ- 

 entiations in connection with syenitic rocks quite different from one another 

 in chemical composition, one being comparatively low in alumina and the 

 other high, the sum of the alkalies in one case being 8.85 per cent and in 

 the other 12.74 per cent. In each instance the associated rocks are facies 

 of one igneous mass. A comparison of the series of differentiation products 

 just described shows to what extent they may differ from one another in 

 neighboring regions. 



