QUARRY MATERIALS OF NEW YORK 1 55 



some of the silica, alkalies etc., would be retained in a condition 

 facilitating their ready migration through any favorable channels 

 that might be formed by the fracturing of the overlying rocks. The 

 formation of pegmatite dikes is thus a normal after-effect of an 

 igneous intrusion. As regards their mineral nature, there seems to 

 be a gradation from a composition about that of granite to very 

 quartzose phases and even to pure quartz. The occurrence of many 

 quartz veins in the vicinity of granite intrusions may thus be ex- 

 plained. 



Forms of pegmatite bodies. Pegmatite intrusions commonly 

 occur in tabular masses which are called dikes when they occupy 

 vertical or highly inclined fissures, or sills if they follow channels 

 in a nearly horizontal plane. Their direction is determined by lines 

 of structural weakness in the country rock, such as faulting, joint- 

 ing and in the case of sediments oftentimes by bedding, whichever 

 structure may afford the easiest outlet toward the surface. Dikes 

 and sills are sharply defined in contact with the country rocks. 

 Though exceedingly numerous in the vicinity of granite masses, 

 they only rarely attain a workable size. Their length naturally 

 exceeds their thickness and it is rather seldom that the latter reaches 

 more than a few feet. 



Of more importance for quarry purposes, at least in this State, are 

 the bosses and stocks of pegmatite that are characterized by a 

 rounded or lenticular form as seen on the surface. Like bosses of 

 ordinary granite, they seem to have made their own outlet toward 

 the surface rather than to have followed some preexisting struc- 

 tural channel. They are more or less irregular in their boundaries, 

 but in a general way approach an equidimensional form as seen in 

 outcrop. They are well defined along the contact with the country 

 rocks. They reach diameters of several hundred feet, as instanced 

 by some of the occurrences in the eastern Adirondacks. They seem 

 to be specially developed in the harder, more massive gneisses and 

 in the granites themselves, whereas dikes occur both in these rocks 

 and in the schists and sedimentary foundations, but are more 

 characteristic perhaps of the latter. 



Besides intrusive pegmatites, there are bodies occurring in the 

 older granites and gneisses which seem to have originated in place 

 by some process of differential crystallization while the magma was 

 cooling ; or in the case of gneisses they may have been found during 

 a period of resoftening of the rock mass incident to metamorphism. 

 They are of varied shape and size, often consisting of narrow bands 

 that shade off on all sides to the parent rock or in large masses 



