PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 47 



a careful study of the quartz of magmatic origin in the Silver Peak 

 district of Nevada. In the Lyon Mountain district, however, the 

 writer finds that silexite masses began to develop not only well 

 before the consolidation of the parent granite magma, but also 

 apparently even before the earliest of the pegmatites. 



It is also a matter of common record that pegmatites are 

 either absent from, or relatively uncommon in, their parent igneous 

 body, while they are relatively abundant in the country rocks. 

 Grout^ has recently cited a number of the many well-known exam- 

 ples. In the Lyon Mountain district, however, pegmatite and 

 silexite masses are extremely abundant in the parent granite, 

 and they are certainly not more common in the relatively small 

 portions of the intruded country rocks which still remain. 



Finally many districts show ordinary pegmatite dikes close 

 to or in the granite, and, traced outward into the country rocks, 

 the pegmatites become more and more siliceous, being practically 

 silexite dikes or quartz veins farthest out. As Pirsson^ says: 

 "Finally passing onward the solution phase might become more 

 pronounced, only siHca would be carried, and the dike turn into 

 a quartz vein." Van Hise,^ in the Black Hills, and Emerson,"* 

 in western Massachusetts, found pegmatite dikes in schist near 

 granite, and farthest out silexite dikes or quartz veins. But in 

 the Lyon Mountain district such a relationship does not hold, 

 since typical pegmatites and silexites in many places occur side 

 by side in the parent granite, and in some cases both form parts 

 of individual dikes or segregation masses. 



PEGMATITES IN THE GABBRO OF THE NORTH CREEK QUADRANGLE 



Some years ago the gabbro masses of the North Creek quad- 

 rangle were discussed at considerable length by the writer.^ The 

 very common if not constant close association of pegmatites with 



' F. F. Grout, Econ. Geol., XIII (19 18), 180-90. 

 ^ L. V. Pirsson, Rocks and Rock Minerals (1909), p. 180. 

 3 C. R. Van Hise, U.S. Geol. Surv., Mon. 47 (1904), p. 724. 

 IB. K. Emerson, U.S. Geol. Surv., Folio 50, 1896. 



5 W. J. Miller, Jour. Geol., XXI (1913), 160-80; N.Y. State Mus., Bull. 170 (1914), 

 pp. 26-28. 



