154 WARREN. 



ous adjustment of equilibrium with falling temperature is assumed 

 to obtain, and, b), for the case where more or less hindered or imper- 

 fect equilibrium is supposed to obtain, chiefly as the result of rela- 

 tively rapid cooling such as might happen in the case of magmas 

 consolidating at, or relatively near, the surface. 



It is concluded that, as held by Vogt, the perthitic structure found 

 in primary potassic feldspar where the amount of albite (plus a little 

 anorthite) does not exceed say about 28%, is due to the unmixing of a 

 pre\'iously homogeneous mixed-crystal. Where more considerable 

 amounts are present it is held that the particular texture which results, 

 whether intergrowth or separate crystals, is very largely determined 

 by the rate of cooling, particularly during the period of initial crystal- 

 lization. Relatively rapid cooling is held to favor in general the 

 development of perthitic structures, or in extreme cases, of homo- 

 geneous mixtures — metastable forms. The presence of such crystalli- 

 zations in a rock is therefore held to be an indication of the relative 

 conditions of cooling, and therefore of the relative geologic position 

 in which the rock magma consolidated. 



It is held, that while the total composition of the perthitic feldspai's 

 of the pegmatites may differ somewhat from those of the granites, 

 the general relations of the two feldspars during crystallization will 

 be much the same in both cases, and that the final composition of the 

 two mixed-crystal phases in both the pegmatites and the granites is 

 approximately represented by the figures found by this investigation 

 on the pegmatitic feldspars. 



It is held that microcline is the stable phase of potassic feldspar at 

 relatively low temperatures, including ordinary temperatures, and 

 that its presence in rocks is much more general than is commonly 

 supposed: its failure to appear is due to a feeble tendency to invert, 

 permitting the inversion interval to be readily passed by under certain 

 conditions. 



Department of Geology, 

 Massachusetts Institute of Technology, Boston, Mass. 

 April, 1915. 



