GEOLOGY OF THE SCHROON LAKE QUADRANGLE 53 



In the small area near the southeastern base of Severance hill, 

 syenite contains numerous inclusions of Whiteface anorthosite. 

 These inclusions are usually only a few feet long and very irregular 

 with their borders more or less assimilated by the syenite. 



Gabbro and Metagabbro(?) 



Distribution, These gabbro and metagabbro ( ?) bodies are, in 

 most respects, very similar to those of the North Creek quadrangle 

 next to the south which have been rather fully described by the 

 writer in his report on the Geology of the North Creek Quadrangle^ 

 and also in the Journal of Geology, volume 21, pages 160-80. On 

 the accompanying Schroon Lake geologic map, twenty-five gabbro 

 masses are represented, all but four of these lying wholly wjthin 

 the quadrangle. They are well scattered over the southeastern 

 two-thirds of the quadrangle, but not one has been found within 

 the area of Marcy anorthosite. A possible explanation of their 

 absence from the anorthosite area is given above in the discussion 

 of the anorthosite. As usual in the writer's experience in the 

 Adirondacks, these gabbro masses appear to have rounded to 

 elliptical ground plans and very steep walls. The variation in 

 length of the areas of outcrop is from one-fifth of a mile or less 

 to 3 miles. In a few of the areas but one or two outcrops could 

 be located. 



A striking feature of the distribution is the fact that the three 

 largest bodies of the quadrangle lie along the border of the Marcy 

 anorthosite. Kemp's Elizabethtown map shows a similar distribu- 

 tion of the largest gabbro masses there. Also on Ogilvie's Paradox 

 Lake map the largest mass of gabbro occurs along the anorthosite 

 border. This remarkable distribution of the gabbro bodies with 

 reference to the anorthosite may be merely a coincidence, or it may 

 have a real significance. If the latter, the writer can think of no 

 very plausible explanation. 



Age. Alost or all of the gabbro is younger than the Grenville, 

 anorthosite and syenite-granite series (i) because of the intrusive 

 contacts against rocks of those series, (2) because dikes of gabbro 

 extend into rocks of those series, and (3) because inclusions rep- 



'N. Y. State Mus. Bui. 170. 



