1 8 Bulletin of Laboratories of Denison University [Voi. xii 



The principal aplite in the Stone Mountain granite is 

 banded with pegmatite, the apUte forming the border next the 

 granite and the pegmatite the middle layer of the dike. The 

 junctions between the granite, aplite and pegmatite are regular, 

 entirely sharp and well defined. Apart from its being more 

 compact and of much finer-grained texture, the aplite is easily 

 distinguished in the hand specimen from that of the inclosing 

 granite by its lighter color — marble white — and by its contain- 

 ing but little mica. Biotite is entirely absent and muscovite is 

 only sparingly distributed through the rock as minute foils. 

 Occasional very small crystals of red garnet are sometimes 

 present. 



In thin section the aplite shows no essential difference in 

 mineral composition from the granite, except in the entire ab- 

 sence of biotite and decreased muscovite. The rock is a 

 holocrystalline mass composed chiefly of the potash (microcline 

 and orthoclase) and soda feldspars, and quartz. Microperthitic 

 structure consisting of interlaminated orthoclase and microcline 

 with a second feldspar, albite, is common. Somewhat irregular, 

 stout laths of a well striated acid oligoclase are numerous. The 

 small percentage of CaO, less than i percent., and the increased 

 Naa O shown in the analysis, column I, indicates the preponder- 

 ance of the soda molecule (albite), which is corroborated by the 

 microscope. Sporadic inclusions of apatite occur. 



Megascopically, the inclosing rock is a compact, medium- 

 grained biotite-bearing muscovite-granite of light gray, nearly 

 white, color. Biotite is only sparingly distributed through the 

 rock, displaying considerable tendency to segregate in places. 

 Thin sections of the granite show quartz, orthoclase, and micro- 

 cline frequently intergrown with albite as microperthite, con- 

 siderable oligoclase, muscovite, occasional biotite, and some 

 prismatic inclusions of apatite. 



The striking similarity between the inclosing granite and 

 aplite is sufficiently shown in the chemical analyses ot the rocks 

 given below. 



The analyses show more Si02 and less K2 O in the aplite 

 than in the granite, with close agreement indicated in the other 



