SIGNIFICANCE OF THE VARIABLE COMPOSITION 409 



This description applies almost equally well to the Adirondack anor- 

 thosite. Most of such gabbroid facies are quite certainly due to local 

 differentiation essentially in situ, with more or less shifting of the differ- 

 entiates during a late stage of magma consolidation. 



Lawson/* in his study of the Minnesota anorthosite, recognizes a small 

 content of femic minerals in the anorthosite in general, and also some 

 dark bands, a few inches to a few feet wide, rich in augite and not sharply 

 separated from the adjacent normal anorthosite. One of these bands dips 

 from 60 to 70 degrees. At one locality he observed a sort of stratiform 

 structure '^due to the fact that there are certain sheetlike layers somewhat 

 richer in pyroxene than the rest of the rock," and he explains this as 

 "essentially^ due to some local chemical differentiation associated with 

 movement in the thickly viscous magma prior to crystallization.'^ 



N. H. WinchelP^ found the Minnesota anorthosite to be much less pure 

 plagioclase and less constant in composition than Lawson, and he states : 

 "Thin sections have revealed in the feldspar boulders, however pure they 

 may be to the eye, small quantities of augite, and from these minute 

 quantities there are all gradations to typically constituted gabbro." This 

 conclusion is confirmed by Elftman as a result of extended field studies. 



Coleman^® says, regarding the largest area of anorthosite in the Eainy 

 Lake region, that toward its western end "the green constituent becomes 

 more important and the rock may be called a porphyritic gabbro." 



The anorthosite-gabbro very commonly, and the typical Marcy anor- 

 thosite less commonly, locally exhibit more or less well developed foliation 

 with exceedingly variable strikes. It is also important to note that granu- 

 lation, which is so prevalent throughout the anorthosite body, shows many 

 extreme variations, often in single outcrops. A striking example of vari- 

 able degree of foliation and granulation is in the big ledge at the south- 

 west end of Moose Island, in Lake Placid, where coarse grained, non- 

 gneissoid, typical anorthosite, with labradorite crystals up to 4 inches 

 across, has in it a zone of much finer grained, granulated, clearly gneissoid 

 anorthosite, the one grading into the other. 



According to Kemp,^^ the anorthosites of the Elizabethtown-Port 

 Henry quadrangle "vary from almost pure aggregates of plagioclase crys- 

 tals through variations caused by increasing amounts of a pyyroxenic com- 

 ponent." He further states that the pyroxene forms from 5 to 15 per 

 cent of the Marcy anorthosite. 



1* A. C. Lawson : Geol. and Nat. Hist. Sur. Minn.. Bull. 8, 1893, pp. 1-23. 

 If' N. H. Winchell : Geol. and Nat. Hist. Sur. Minn., Bull. 22, 1893, pp. 18-19. 

 18 A. P. Coleman: .Tour. Geol., vol. 4, 1896, p. 911. . 

 " J. F. Kemp : N. Y. State Mus. Bull. 138, 1910, p. 28. 



