APLITE, PEGMATITE, AND TOURMALINE 187 
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 dis- 
tinguished in the hand specimen from that of the inclosing 
granite by its ighter 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 
absence 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 1 per cent., and the increased 
Na,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 oliogclase, 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 of the rocks 
given below. 
The analyses show more SiO, and less K,O in the aplite 
than in the granite, with close agreement indicated in the other 
constituents. A striking feature of the analyses is the low 
