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NEW YORK STATE MUSEUM 



anorthosite area occupies a central position in the Adirondacks. The 

 area within the Paradox Lake quadrangle comprises a segment of 

 the outer portion of the intrusion, and in it a series of meta- 

 morphic changes are evident. These will be described in part 4, on 

 petrography. 



The anorthosite forms the highest and most rugged mountains 

 in the quadrangle. 



Gabbro. In a few small patches gabbro rocks occur — true gab- 

 bro, norite, and hornblende gabbro (meta-gabbro). The most im- 

 portant exposure is on Peaked hill. 



The gabbro is typically a medium grained rock, of greenish black 

 color. The dark color serves to separate the rock from the anor- 

 thosite and syenite. 



Several dikes of the gabbro are found. On the southern shore of 

 Pharaoh lake a gabbro dike cuts gneiss ; on Bull Rock mountain 

 (called Old Fort on the map) a gabbro dike cuts the syenite; two 

 dikes southwest of Chilson cut graphitic sandstone ; one dike about 

 a mile west of Chilson cuts sedimentary gneiss ; on Moose moun- 

 tain a very basic gabbro dike cuts anorthosite. 



The gabbros can in many localities be traced directly into horn- 

 blende schists, or amphibolites, and there is no doubt that many 

 dikes of these g'abbros exist. But in some localities, as on Ellis 

 mountain in Hague, dikes are exposed which are purely schistose, 

 with no trace of massive facies. It becomes a matter of some diffi- 

 culty to determine whether such dikes belong to the syenite in- 

 trusion, or to the gabbro, or whether they represent a distinct 

 intrusion in themselves. It is upon the age of these dikes that the 

 relative ages of the syenite and granite depends. The granite is 

 frequently cut by dikes of hornblende schist ; if these could be proved 

 to belong to the syenite the relative age would be established. In 

 the localities on the south shore of Pharaoh lake and on the moun- 

 tain west of Goose pond these schists occur, grading directly into 

 massive gabbros. In the last-mentioned locality are a series of dikes 

 of pure schist, precisely like these questionable ones which so fre- 

 quently cut the Pharaoh gneiss, but fortunately in one a massive 

 facies was found which placed this set with the gabbros. It is 



