336 



anorthite, these latter being selectively locked up in the inner 

 zones of plagioclase. 



The non-volatile constituents of the residual magma thus 

 consisted essentially of quartz and microcline and subordin- 

 ate^ slightly calcic albite. 



With the renewal of crystallization in the residual magma 

 quartz and microcline were early precipitated, the magma 

 being thus constantly depleted in these constituents. That 

 some albite crystallized during this period is also evidenced by 

 the presence of subordinate albite, associated with the micro- 

 cline and quartz. This albite was still slightly calcic and with 

 ■crystallization its composition approached pure albite. The 

 amount crystallizing, however, was quite subordinate, and the 

 impoverishment of the still liquid residue in microcline 

 molecules especially occurred. 



Nearing the completion of crystallization of such a magma 

 pool, the residual liquid would have markedly changed in 

 composition through such selective crystallization. This 

 residual liquid depleted in potassic constituents would there- 

 fore have become highly sodic. 



By the opening of fissures in the surrounding rock this 

 residual liquid, derived by fractional crystallization from the 

 dominant potassic magma, was strained off from the crystalline 

 mass and solidified in the occupied fissures. 



Derived by such a process of fractional crystallization this 

 residual liquid would be: — (i.) Predominantly sodic ; (ii.) 

 characterized by an increased concentration of mineralizers. 



This process of differentiation receives considerable sup- 

 port from a study of the albitites. 



Their coarse-grained texture and the concentration in 

 them of such minerals as zircon, apatite, and rutile are 

 characteristically to be associated with a concentration of 

 mineralizers during crystallization, or, in other words, they 

 are typical end products of differentiation. 



It has been noted that some muscovite and quartz in the 

 muscovite-albitites appears to be secondary. This is prin- 

 cipally evidenced by the shattering of albite plates by small 

 quartz stringers. This pneumatolytic process is intimately 

 related to the concentration of mineralizers during the 

 crystallization of the rock. The shattering of albite plates 

 by quartz strings and the production of muscovite can be 

 relegated to a late stage of crystallization, i.e., at or near 

 the completion of crystallization. The muscovite, indeed, 

 may represent the hydrolysis of potential microcline-felspar. 



