l6o NEW YORK STAT.E MUSEUM 



There is considerable uncertainty as to the quantity of available 

 material in pegmatites, even when they have been well exposed at 

 the surface. Unlike normal granites, they are very liable to sudden 

 variations in the proportions and relations of the quartz and feld- 

 spar, such variations arising quite abruptly. This involves a con- 

 siderable element of risk, particularly in the working of small 

 bodies for some particular grade of feldspar. In the larger dikes 

 and bosses, the desired quality may be obtained by carrying on 

 work in several places and sorting the product carefully into grades. 

 Thus at Bedford three grades of feldspar are produced from one 

 body, besides a quartz by-product. With a small output, it is not 

 practicable always to sort the product so carefully and there is 

 consequently more waste. 



THE LOCAL DISTRIBUTION OF PEGMATITES IN NEW YORK 



STATE 



The pegmatites are limited in their occurrence to the two prin- 

 cipal areas of early crystalline rocks represented by the Adiron- 

 dacks and the southeastern Highlands. They occur in the vicinity 

 of the larger granite intrusions, but the workable bodies are more 

 often found on the periphery of such intrusions and within the 

 older country gneisses and schists than in the midst of the granites 

 themselves. They appear sometimes in the areas where ordinary 

 granites do not outcrop, but in this case they may be offshoots of 

 some 'buried mass that were able to reach the surface on account of 

 their fluid condition. 



The Adirondack region is well supplied with pegmatites, but they 

 are by no means equally distributed. The great anorthosite mass 

 that spreads over the eastern central part, mainly within Essex 

 county, is naturally devoid of occurrences, as it is of later intru- 

 sives generally, except those of basic character. In the fringe of 

 gneisses to the east of that mass there are granite intrusions and 

 pegmatites, some of the latter of large size, as those near Crown 

 Point and Ticonderoga. In the northern Adirondacks, which is 

 largely occupied by a belt of very old gneisses, few intrusions of 

 younger granite are encountered. So far, only one large pegmatite 

 body has been reported in that section. The southern Adirondacks 

 have a number of occurrences and it may be expected that others 

 will be found here as the region is more carefully explored, but 

 they are likely to be in the more inaccessible parts. The western 

 Adirondacks, particularly the section included in St Lawrence, 

 Jefferson and northern Lewis counties, is known to include numer- 



