THE PROBLEM OF THE ANORTHOSITES 219 



be injected as small dikes. If it is considered that about 50 per 

 cent liquid is necessary, then only anorthosite or, better, anorthosite- 

 gabbro, with about 85 per cent plagioclase could occur as small 

 dikes. If somewhat less than 50 per cent liquid is necessary, then 

 a rock somewhat richer in plagioclase could occur in that manner. 

 In the case of effusive masses, if it is considered that more than 50 

 per cent liquid is normally requisite for their formation, only anor- 

 thosites with less than 85 per cent plagioclase could occur as 

 effusives. 



A study of the literature of anorthosites from various localities 

 seems to show that, in so far as published descriptions are con- 

 cerned, anorthosites do have substantially the characters outlined 

 in the foregoing discussion, which is based on the hypothesis that 

 they are accumulated masses of plagioclase crystals. Still the idea 

 is rather novel, and probably no one had such a hypothesis in mind 

 when examining anorthosites, so that, while many observations 

 bearing directly on the problem are recorded, one might readily 

 believe that perhaps many others equally pertinent escaped record. 

 For this reason the writer spent a few weeks in the Adirondack area 

 and in the Morin area of anorthosites, becoming acquainted at first 

 hand with the relations there found. Attention was confined 

 almost entirely to parts already mapped in detail so that a maxi- 

 mum could be seen in the limited time. The facts bearing on the 

 origin of anorthosites in these areas will be stated principally as 

 recorded by others, and only to a very limited extent supplemented 

 by this brief personal experience. It is desired to express thanks to 

 Professors Kemp, Gushing, and Adams and to Mr. Dresser for 

 interest taken and for furtherance of the work in various ways. 



THE ADIRONDACK ANORTHOSITE 



General relations. — The anorthosite of the Adirondacks occurs 

 principally as a single great area, for the most part in the heart of 

 the mountains and making up its highest peaks, though extending 

 eastward to the lower country in the vicinity of Lake Champlain. 

 The mass occupies an area approximating 1,200 square miles, the 

 principal constituent of the rock throughout this area being 

 plagioclase. Large exposures may be made up almost exclusively 



