20 GOLD AND TIN DEPOSITS OF SOUTHERN APPALACHIANS. 
IGNEOUS ROCKS. 
The locks described under this section are (hose which were intruded and reached theii 
present position after most or all of the metamorphism which affected the rocks descrtbei] 
in the foregoing pages had taken place. They exist at the present (jay, tl Tore, in prac- 
tically the same state in which they crystallized from their molten magmas. These roc£| 
have a small area! extent as compared with the met amorphic rocks. They appear eithei 
as dikes or as elongated bodies, whose shape was influenced by the structure of the rocks 
which they penetrated. 
Granite is the most abundant of the massive igneous rocks. It is <f light color/medium 
grain, and typical granitic texture. Exposuresof large size someti s show the rock to be 
almost structureless that is, devoid of any foliation — but in rr places a more or less 
distinct gneissic structure is observable, in spite of the fact that 1 -id specimens may appear 
nearly or quite massive. This granite is rich in the lighter-coi >red constituents, which 
include muscOT ite, and contains a rat her sparing amount of brown biotite. 
The microscope shows the presence of abundant quartz in clear grains, holding numberless 
fluid inclusions of the ordinary kind. Microcline is the predominant feldspar, followed in 
decreasing amounts by orthoclase, oligoclase, and albite.all fresh. Muscovite is plentiful 
in broad, thin foils. Deep red-brown biotite is not very abundant. By its bleaching, some 
of the muscovite may have been formed. Some of it has been changed to chlorite. Scat- 
tered garnets, a few crystalline grains which are probably epidote, one or two small zircon 
crystals, and numerous prisms of apatite complete the list of constituents. 
A comparison of this description with that of the , unit ic gneiss (pp. 16-17) in the sect ion on 
i net amorphic rocks shows thai the tun types of rock are practically identical mineralogieally. 
While the Held reflations are not very distinct and have not been carefully studied, it is 
believed that they likewise favor the view that the foliated granite or gneiss and the massive 
granite were derived from a common magma and belong to the same general period of 
intrusion. If this is true then the eruption of the granite must have marked the closing 
stage of the regional metamorphism of the country; otherwise, all the granite would be 
foliated. The different n amount of foliation of the granite and the gneiss may partly 
be explained by the r< .v'-hle supposition that these last metamorphic movements were; 
not equally intense i portions of the area. It seems probable, however, that other 
conditions were 1 largely ..sponsible for this difference in foliation. 
It has just been stated that the granite and the gneiss probably belong to the same. 
genera] period o f intrusion. T - does not mean, however, that all the masses of these) 
rocks vpere intruded at the same absolute time. It is doubtful, indeed, if instances of su^H 
kind ar .a I common. On the other hand it is reasonable to suppose that successive 
intrusions took place and that the interval from the beginning to the end of eruptive acti\ ity 
.as considerable. Since this epoch of eruption probably began just as the great movements 
of compression a ^folding were dying out, the fust of the granitic intru ions would suffer 
mote or less M|u«e -ng and shearing and would be converted into a granitic gneiss, while 
successively later portions of the granite magma to be forced up into the metamorphose! 
rocks would be subjected to less and less dynamic action, and the last of the granite would 
appeal' after metamorphic movements had ceased, and would thus remain massive. 
PEGMATITE. 
The dikes of pegmatite which abound in the western part of the area studied form one of 
the most interesting geologic features of the region. Several varieties of pegmatite may 
be distinguished, although it may be that all are members of a series in which certain 
varieties are most common or abundant. Of the types established several exhibit a cool 
siderable variation in mineralogical composition, so that it is not at all improbable that a 
further slight change would result in a passage to the neighboring type. Although these 
rocks are of various appearance's, they all possess features which at once characterize them 
