LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 47 



molecules and water to give a high concentration and consequent 

 separation of some of those molecules which are formed by the 

 breakdown of the alkaline feldspar molecules (biotite or quartz or 

 both). Thus may originate a quartz-diorite as a salic differentiate, 

 or with still slower cooling a granodiorite or a granite. When 

 cooled somewhat too quickly to allow appreciable settling of crys- 

 tals, zoning of plagioclase crystals may bring about the same effect 

 on the liquid, and the saHc ty^ts mentioned above may occur as 

 interstitial material. 



From some basaltic magma the early formation of olivine may 

 take place, with the result that the amount of possible sahc differ- 

 entiate is augmented. This early separation of oHvine may occur 

 during the slow coohng of a magma which with quick cooling gives 

 no olivine but only plagioclase and pyroxene. Further, from some 

 basaltic magma, which even with quick cooHng gives considerable 

 olivine, spinel may be formed among the products of early crystal- 

 lization when the cooHng is slow (see p. 30). Peridotite or spinel- 

 bearing peridotite may, therefore, be formed by collection of the 

 early crystals and the salic differentiate be thereby increased. 



The crystallization of a normal basaltic magma always tends 

 toward biotite granite, but the biotite granite stage is reached 

 only with slow coohng. With somewhat quicker cooling, the 

 process of sinking of crystals cannot keep pace with the quicker 

 using up of the liquid, owing to more rapid crystaUization. Crys- 

 tallization and therefore differentiation becomes complete at the 

 granodiorite, quartz diorite, or diorite stage according to the rate 

 of cooling. It is clear that a dioritic border phase occurring about 

 a granite or granodiorite mass is to be attributed to the fact that 

 cooling at the border was maintained considerably in advance of 

 that farther away from the border, and not to any diffusion of 

 basic material toward the cold wall. In the cooler outer portion, 

 crystallization was complete at the diorite stage, but in the warmer 

 interior, differentiation by removal of crystals continued till the 

 later stage of granodiorite or granite.^ Daly's explanation of basic 



' B. S. Butler has come to this conclusion in the case of a great number of stocks in 

 different sections of Utah (personal communication from Mr. Butler shortly to be 

 published in Economic Geology) . 



