PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 45 



2. The Lyon Mountain granite contains a great profusion of 

 silexite and pegmatite masses, while the Hawkeye granite con- 

 tains relatively few. 



3. The Hawkeye granite contains many aplite dikes, but none 

 at all was observed in the typical Lyon Mountain granite. 



4. By far the greater number of the pegmatites, both the earlier 

 and the later types, consist almost wholly of quartz and alkalic 

 feldspar. With a few local exceptions minerals which indicate 

 the former presence of mineralizers other than water vapor are 

 practically absent. 



5. Silexite masses began to develop, probably as segregation 

 products, while the granite magma still possessed a very consider- 

 able degree of fluidity, and they continued to form probably both 

 as segregation products and as dikes, until the inclosing granite 

 almost, or possibly completely, solidified. 



6. Pegmatite masses also began to develop well before the 

 solidification of the magma, though probably not as early as the 

 earliest silexite masses, and they continued to form until almost, 

 or possibly complete, consolidation of the inclosing granite. 



7. The silexite masses represent very siliceous facies of the 

 pegmatitic development, gradations from nearly pure silica to 

 ordinary pegmatite being not uncommon. 



8. By far the greater number of the pegmatites, silexites, and 

 aplites occur within the parent granites, though some also occur 

 in the relatively small remnants of the rocks into which the gran- 

 ites were intruded. 



9. The aplites were developed during a late stage of the 

 magma consohdation, none as early as the earliest pegmatites and 

 silexites, and probably none as late as the latest pegmatites. 

 Some apHtes cut others, thus proving intrusions distinctly different 

 in time. 



Authors of practically all the standard works on petrology 

 definitely regard pegmatites as intrusions into the already soHdified 

 portions of the magmas from which they were formed. Iddings^ 

 says that pegmatitic liquid may be made to "flow into openings 

 formed by the Assuring of completely solidified parts of the magma, 



''J. P. Iddings, Igneous Rocks, I (1908), 272. 



