488 NEW YORK STATE MUSEUM 



minerals,^ and is a huge pegmatite which has been opened for the 

 economic value of the orthoclase in the manufacture of porcelain. 

 Crystals of biotite, orthoclase and quartz, sometimes over a foot in 

 length, occur at this quarry. Three small diabase dikes cut it, along 

 which are developed tourmaline and titanite. There are several 

 good sized hills consisting of pegmatite in this locality, with smaller 

 pegmatites cutting sedimentary gneiss. 



This famous pegmatite can be followed towards the granite 

 intrusion, with increasing biotite as the granite is approached. 

 When followed away from the granite the pegmatite becomes more 

 acid and contains much graphic granite. Small dikes of pegmatite 

 border the mass around the spar bed, these dikes being more acid 

 than the larger ones. Beyond the dikes are veins of rose quartz. 



Many pegmatites border the granite of Mount Pharaoh. In this 

 case small dikes only were found. Those near Mount Pharaoh 

 presented the general mineralogy of a granitite, usually with acces- 

 sory tourmaline or titanite; the more remote ones contained fewer 

 dark silicates. 



Pegmatites also occur about the edge of the anorthosite area. 

 These pegmatites contain the same bisilicates as the anorthosite, with 

 quartz, orthoclase and magnetite also. 



There seems no doubt that these pegmatites belong to the closing 

 stages of the intrusions ; and that they are of igneous origin, but were 

 produced with the aid of more water than their associated plutonics. 



On the hills about Crane pond pegmatites are particularly abun- 

 dant near the limestone contacts, while quartz veins predominate in 

 the quartzose gneiss area. There are complete gradations between 

 the two, though any connection with plutonic sources is here cut off 

 by faults. Professor Van Hise has pointed out^ that the true explan- 

 ation of pegmatization includes igneous injection, aqueo-igneous 



^ Roe's spar bed has been referred to in the following papers: E. H. Williams, 

 Am. Jour. Sc. 1881, on tourmaline; J. F. Kemp, Am. Jour. Sc. 1888, on 

 minerals near Port Henry; J. F. Kemp on Geology of Crown Point, N. Y. 

 N. Y. State Geol. An. Rept for 1893 ; J. F. Kemp, U. S. Geol. Survey Bui. 107. 

 on Trap Dikes of Lake Champlain. 



^C. R. Van Hise. Principles of Precambrian Geology. U. S. G. S. i6th An, 

 Rept pt I. p.684-687. 



