24 NEW YORK STATE MUSEUM 



body of anorthosite once possessed fluidity enough to permit dis- 

 tinct magmatic currents or movements. The significance of the 

 foh'ation is thus an important consideration. Even the typical 

 Marcy anorthosite, almost entirely free from femic minerals, not 

 rarely exhibits a magmatic flow-structure foliation (plate 6), the 

 labradorite crystals having been strung out into crude parallelism 

 in a yet molten portion of the rock. Not only was this interstitial 

 liquid in sufficient quantity to permit the development of distinct 

 magmatic flowage, but it was essentially molten plagioclase. It 

 could have been nothing else. Hence we here have evidence directly 

 opposed to Bowen's statement that the anorthosite was never at a 

 temperature sufficiently high to melt plagioclase. It is not argued, 

 however, that the anorthosite as such necessarily was intruded 

 in the form of a true magma to its present position, having been 

 differentiated at a much lower level. Rather, it is probable that 

 a gabbroid magma was the original intrusive which, either during 

 the process of intrusion or after the magma came nearly to rest, 

 or both, differentiated to give rise to the anorthosite which was 

 then, in considerable part at least, really molten. This matter is 

 more fully discussed below. 



Though the writer believes the anorthosite as such to have been 

 molten to a very considerable degree at least, it is by no means 

 necessary to assume that it was ever completely molten with a high 

 degree of fluidity, or even only a moderate degree of viscosity. 

 None of the field facts, however, necessarily preclude the hypothesis 

 that the whole mass of the anorthosite may once have been com- 

 pletely molten, but without a high degree of fluidity. 



Before leaving this consideration of the significance of the varia- 

 bility of the anorthosite, emphasis should be placed upon the fact 

 that, in many places, its mass shows unmistakable evidence of 

 having differentiation phases of anorthosite-gabbro or even gabbro, 

 while there is no positive evidence for its differentiation into sye- 

 nite or granite as should be the case according to Bowen's 

 hypothesis. 



Relation of Whiteface and Marcy types of anorthosite. In the 

 Schroon Lake quadrangle as elsewhere, it is clear that the White- 

 face anorthosite is a gabbroid border facies -of the Marcy anor- 

 thosite with perfect gradations from one into the other, and with 

 ho evidence that syenite or granite was ever developed as a rock 

 intermediate between the border phase and the true anorthosite as 

 required by Bowen's hypothesis. Though it has been notably cut 

 into, and partly assimilated by the syenite-granite body, a glance at 



