GEOLOGY OF THE LAKE PLACID QUADRANGLE 29 



5 Certain areas of mixed gneisses are anorthosite literally cut 

 to pieces by syenite, often with fairly sharp contacts visible. Such 

 a relationship is anything but stratiform as conceived by Bowen. 



6 The anorthosite is by no means an almost perfect homogeneous 

 mass of plagioclase. Most of the rock contains from 2 to 5 per 

 cent of minerals other than plagioclase ; portions with 5 to 10 per 

 cent are not uncommon ; and sometimes the rock contains 10 to 20 

 per cent, or even more, of dark minerals. Such gabbroic facies 

 exist locally throughout the anorthosite body, sometimes as narrow 

 zones or belts. On the basis of the origin of the anorthosite by 

 the settling of plagioclase crystals, how are such variations to be 

 accounted for? Also, since so much of the rock contains very 

 appreciable amounts of ferro-magnesian minerals, is the mutual 

 solution theory necessarily precluded? 



7 Foliation, particularly of the Whiteface anorthosite, is by no 

 means rare, and the writer has repeatedly seen highly gneissoid 

 facies and facies with little or no foliation in close proximity. The 

 writer believes that the alternations of gabbroic and nongabbroic 

 facies, and gneissoid and nongneissoid facies, of the anorthosite 

 are not results of regional compression, but that they were devel- 

 oped essentially by forced differential flowage in a congealing 

 magma. This matter is explained at some length beyond under 

 the caption " Foliation." 



Some of the phenomena above described might possibly be har- 

 monized with Bowen's hypothesis that the anorthosite originated 

 by the " accumulation of plagioclase crystals precipitated from 

 solution in a mixed magma," but, taken altogether, the writer 

 believes that they render such an hypothesis untenable. 



Relation of Whiteface anorthosite to Marcy anorthosite. As 

 above stated, it is often a matter of judgment as to where the bound- 

 ary lines between the Whiteface and Marcy anorthosites should 

 be drawn. As a result of the field studies, the best evidence points 

 to the conclusion that the two types are merely differentiates of 

 the same cooling magma. On the one hand, in spite of many careful 

 observations in the field in passing from one type across to the 

 other, no evidence was obtained to show that one type cuts the 

 other, while, on the other hand, one type of the rock grades into 

 the other in many places. Transition rocks in some places form 

 zones only some rods in width, while in other places they may be 

 one-fourth of a mile across, and then any accurate delimitation 

 of the Whiteface and Marcy types on the geologic map is impos- 

 sible. Among many localities where intermediate facies are well 



