IGNEOUS ROCKS. - i 
■ 
as pegmatites. In composition they are all closely: related to granite. In the following 
paragraphs the four most common varieties are des ribed in the probable order of their 
formation. 
The first variety is usually of light color and Sol v<. fry cparse grain. It occurs in small 
contorted lenses and dik°s in the granite and surrounding rocks. Microcline, quartz, 
oligoclase, orthoclase, and muscovite are present in decreasing amount. Small red garnets 
are numerous in some specimens and ilmenite is a prominent constituent of some of the 
small masses. Crushing can be seen to have taken place in much of the rock. In some 
cases it appears as if contortion had gone on while the pegmatite was unsplidified. 
The pegmatite of the second variety is a light-colored rock, much of it somewhat foliated 
or gneissic, whose chief constituent is feldspar. The microscope shoves large individuals of 
orthoclase with well-developed cleavage, some of them with iuieropcrthitic intergrowths of 
albite. Numerous smaller grains of albite also occur near the periphery of the orthoclase 
individuals. Narrow tongues of quartz appear to be eating into the feldspars and are 
surrounded by small grains of micropegmatite — a granophyric intergrowth of quartz and 
feldspar. Small shreds and scales of brown biotite are plentiful in some parts of the section. 
and associated with the biotite particularly are small prisms of monazite. In thin 
section this mineral is pale yellow, and has a high index of refraction and apparently rather 
low double refraction. In many places the dikes contain flakes of graphite, whose origin 
is undoubtedly primary. 
Dikes of this kind are rather common in the western part of Cherokee County, where 
they are injected into gneiss and amphibolite, and where, on sufficient decomposition, they 
are sometimes worked for monazite. 
The third variety of pegmatite is very light colored and fairly coarse grained. In most 
places quartz and feldspar are the only constituents visible in the hand specimen, but in 
some places muscovite is present and locally cassiterite becomes an essential constituent. 
It is rather easily decomposed and weathers to a soft mass chiefly made up of kaolin. 
Microcline is abundant in large individuals penetrated by grains of albite and by long, 
narrow tongues of quartz, which look t»s if they had eaten their way into the feldspar. Along 
the borders of these larger quartz individuals are small grains of different orientation. The 
quartz is crowded with fluid inclusions. Much of the muscovite is converted into the fibrous 
variety, sericite, by crushing. Cassiterite, when present, occurs as crystalline grains of 
dark color and pleochroic under the microscope. This kind of pegmatite, is known only in 
the vicinity of the Ro%* tin mine, near Gaffney, Cherokee County, S. C. In some respects 
jfeit resembles the monazite-bearing pegmatites from the western part of Cherokee County, 
|£&£l the occurrence of a notable amount of monazite in the bed of the stream which crosses 
pr^W dikes of the Ross property may be an indication that this variety also i^ monazite 
- bearing. Neither biotite, monazite, nor graphite has been found in the Ross dikes, however^ 
A slight change in composition of the pegmatite of this class would probably result in 
the production of the fourth variety. 
The fourth kind of pegmatite consists very largely of quartz and muscovite. It is 
coarse grained, some of the mica flakes attaining a width of 10 cm., and in this way merits 
the name "giant granite" sometimes given to it. 
As a rule feldspar is absent or only sparingly present, but in certain cases good-sized 
crystals of feldspar are rather abundant. Where these dikes penetrate the metamorphic 
rocks they seem in general to contain much mica and Ktile or no feldspar. Where they 
cut the granite or granitic gneiss, on the other hand, they usually hold a plentiful amount 
of feldspar and decidedly less muscovite. Cassiterite is in f ve places an important con- 
stituent and seems to be most abundant in the mica-rich varieties. Among other minerals 
which have been found in these dikes are large but imperfectly terminated crystals of 
^podumene, violet-blue or bright-green apatite, and a brownish or yellowish-brown lithia- 
manganese phosphate , probably lithiophilite. These dikes are in some places so resistant 
to atmospheric agencies as to outcrop conspicuously. Many of them arc very friable, 
however, and hence disintegrate and are concealed by soil. This variety of pegmatite is 
