330 



NEW YORK STATE MUSEUM 



dike are found on both sides of the river, showing both the ophitie 

 and the granular types of the rock; a small boss on the north- 

 west shore of Upper Chateaugav lake, which shows ophitie 

 gabbro in the center passing rapidly into an amphibolite gneiss 

 on all sides; and a considerable boss not far from the lower end 

 of Chazy lake. These all show portions only slightly metamor- 

 phosed and still retaining ophitie structure. There are many 

 other places in the county where wholly gneissoid rocks of gabbroic 

 make-up occur as dikes and are in all likelihood referable to this 

 same group. In Franklin county there is a considerable boss by 

 the north branch of the Saranac, 2 miles east of Hunters Home, 

 showing a fairly coarse rock with a comparatively unmetamor- 

 phosed core; there is another, well shown in cuts along the New 

 York & Ottawa Railroad, 2 miles above. St Regis Falls, which 

 shows beautifully the gradual passage from the unchanged core 

 into amphibolite, the latter containing a profusion of enormous 

 garnets; and there is another showing along the west shore at 

 the upper end of Lower Saranac lake, which is quite a large mass 

 and correspondingly coarse, and which must cut the anorthosite, 

 since it is surrounded by that rock on all sides, though no con- 

 tacts were seen. There are here also numerous smaller masses 

 and dikes which are more completely metamorphosed. Kemp 

 and Smyth have shown the wide distribution of similar rocks in 

 the eastern and western Adirondacks. 



To gabbros of this type, with ophitie structure, the name 

 " hyperite " has been applied by Tornebohm. 



In many localities metamorphism has produced an amphibolite 

 from these gabbros, instead of the merely granulitic gabbro. In 

 these amphibolite phases there is always considerable pyroxene 

 in addition to the hornblende, but in the field these are absolutely 

 not to be distinguished from the amphibolites associated with the 

 various gneisses, and these too often contain pyroxenes. Where 

 there is an unmetamorphosed core, the origin and relations are 

 evident, otherwise they are wholly obscure. This gradation is 

 beautifully shown at the localities on Upper Chateaugay lake 

 and along the New York & Ottawa Railroad referred to above. 



The fact that these gabbros are found in dikes cutting the 

 anorthosites shows conclusively that they are younger. The 

 writer has noted gabbro dikes also cutting the syenite in the Little 



