102 NEW YORK STATE MUSEUM 



THE SPLIT ROCK ANORTHOSITE AREA 



The great anorthosite intrusion of the central Adirondacks has its 

 most easterly exposure on Split Rock mountain, the bold ridge that 

 forms the western shore of Lake Champlain for several miles, be- 

 ginning just north of Westport. The whole mountain is practically 

 made up of this rock and its gabbroic type, though on the north 

 end it gives way in places to the Grenville series of limestones and 

 schists which have been surrounded and borne up apparently by the 

 igneous mass. The darker phase of the anorthosite is mainly in 

 evidence in the exposures along the lake and on the north end. 

 The bulk consists of the grayish feldspathic variety which has been 

 more or less comminuted by regional compression. In some parts 

 of the mountain the rock has a distinctly porphyritic appearance by 

 reason of the large residual feldspar crystals, but again it shows a 

 local development that is characterized by uniformity of grain. 



The only quarry workings in this exposure that are known to the 

 writer are on the eastern face of the mountain, about one-fourth 

 of a mile back from the lake and at an elevation of from 500 to 

 600 feet. They are reached by a trail from the Westport road and 

 also from the lake by following the old tramway line that was 

 used to lower the stone. The locality is just north of the little bay 

 called Barn Rock harbor on the United States geological sheet, but 

 is mentioned as Barron Rock in Smock's report of 1888. According 

 to the latter, the quarries were first opened in 1881 by the Cham- 

 plain Granite and Marble Co., and reopened in 1887 by the Adiron- 

 dack Granite Co. Under the latter company, as the writer has been 

 informed, a quantity of building and monumental stone was shipped, 

 some of the building material having been sent to New York City. 

 By 1890 the quarries were again closed and have not been worked 

 since. 



General characters. The stone from the quarry site has a 

 grayish body with porphyritic feldspar of somewhat darker color. 

 It is practically all feldspar, belonging to the very basic plagioclase 

 series. Small, scattered crystals of pyroxene (diopside), magnetite 

 and quartz occur in the interstices of the feldspar aggregate. The 

 magnetite shows slight decomposition to hematite, but there is little 

 pyrite, judging from the samples that were examined. 



DANNEMORA GRANITE AREA 



A gneiss of massive granitic appearance, pink or gray in color, 

 outcrops on the ridge north of Dannemora, Clinton county. The 

 exposure is a part of the larger belt of granitic and syenitic gneisses 



