Cushing — Geology of Clinton County. 



509 



group in the Adirondacks, is in Essex county, where they have a greater area] 

 extent than all the other rocks combined, but they pass beyond the borders 

 of that county on all sides. Three outlying areas of this group occur in 

 Clinton county, none of which has been heretofore recognized as belonging 

 to it. The first of these is in Ausable township, extending north and west of 

 Keeseville, and is the direct prolongation northeastward of a great gabbro 

 ridge which comes up to Keeseville from the southwest. The second forms 

 Rand's hiil in Beekniantown and Altona townships, is twenty miles north of 

 the last, and interesting as carrying the known Adirondack exposures by that 

 distance nearer to the Canadian ; further connecting links, if any, being 

 concealed by the covering of the later rocks beneath which they pass. The 

 third area forms the Catamount mountain ridge in southwestern Black Brook, 

 and Professor Kemp's work during the past season indicates that the Whiteface 

 massive, just over the border to the southwest, is a prolongation of the same. 

 The Catamount mountain exposures are difficult of access, and have not been 

 so thoroughly studied on that account; but the rock, while a little abnormal, 

 is unmistakable gabbro. 



In their typical development the Adirondack gabbros, like those of 

 eastern Canada, consist almost wholly of plagioelase feldspar, mostly labra- 

 dorite, but ranging to anorthite, and to this very feldspathic phase Dr. F. D. 

 Adams has applied the name anorthosite. As the peripheries of the masses are 

 approached, ferro-magnesian silicates appear in increasing quantity, and ulti- 

 mately the gabbros may become very basic, titaniferous magnetite, augite and 

 sometimes hypersthene being present in large quantity, together with garnet, 

 hornblende and biotite, these latter being largely, if not wholly, secondary. 

 With the appearance of these minerals comes almost always a gneissoid 

 structure in the rock.* 



Though the Clinton county gabbro areas are of comparatively slight 

 extent, quite typical anorthosite may be collected from each, occurring in close 

 association with basic gabbro. Sometimes the ordinary gabbro contains 

 narrow bands of a quite basic character, of such persistence and with such 

 sharply denned walls that they closely imitate dikes in appearance. It is 

 more especially such bands as these, or larger masses of the same character, 

 that are quite identical with the bands already mentioned as occurring as basic 

 gneisses in the gneiss series, and which are to be regarded as dikes or apophysae 

 of the main gabbro intrusion which penetrated the gneiss from the parent 



* For full descriptions of some phases of the gabbros, see F. D. Adams, Neues Jahrbuch f6r Min , Vol. VIII.. pp. 419-497 ; 

 J. F. Kemp, Bulletin Geological Society of America, Vol. V., pp. 213-224. 



