OF SOUTH CAROLINA. 9 
Gneiss forms an important rock in the principal mountain ranges of the world, and asa metallif- 
erous rock, few of the metamorphic rocks are of equal consequence. Many of the principal mines 
in Norway, Sweden, Saxony and Bohemia occur in gneiss. 
Hornblende slate.—Alternating with gneiss, we find strata having a dark or greenish color, which, 
on examination, is found to result from the substitution of hornblende for the mica of the gneiss. 
It is often difficult to distinguish this rock, by external appearance, from those varieties of gneiss 
with black mica. The easy cleavage of the mica, however, will always be sufficient to distinguish 
the two. 
From the trap rocks, which it may resemble, it can always be distinguished by its slaty structure ; 
the hornblende and feldspar, of which it is generally composed, being disposed in alternate layers. 
Lime rock.—Beds of lime rock are also of common occurrence in gneiss. Although frequently 
crystaline in structure, they are generally stratified, having layers of mica disposed in planes, along 
which the rock splits readily. Lime rock occurs in these beds of every degree of purity—from 
gneiss, containing a little calcareous matter, to pure lime rock. A granular variety, containing mag- 
nesia, is known by the name of dolomite. 
Mica slate, which is composed essentially of quartz and mica, passes insensibly into the other 
slaty rocks with which it is associated. Into gneiss by the addition of feldspar ; into talcose slate 
by the substitution of tale for mica; and when the quartz disappears, the mica is seen in small 
scales; it then passes into clay slate--so that, where these rocks occur together, it becomes 
extremely difficult, if not impossible, to define their limits. Although the rocks just mentioned 
often alternate, the mica slate generally occurs in the order in which I have placed it, resting on 
the gneiss. 
Quartz rock is, in texture, generally granular, the grains presenting every degree of coherency, 
from that of loose sand to the solid rock. It requires the examination of this rock in masses of 
considerable extent, in order to distinguish it from common quartz, as it occurs in veins. In South 
Carolina it is always stratified, and more or less mixed with mica or tale. The passage from the 
arenaceous to the compact variety is gradual, and it is in this passage that it assumes the form of 
itacolumite, a variety that has assumed some interest from its connection with the diamond-bearing 
rocks of South America. 
Another curious form of this rock occurs in South Carolina: it has the appearance of a conglo- 
merate, on the weathered surface, but this structure is not presented by a fresh fracture. It seems 
to be the result of the manner in which iron is distributed through the mass, and which is only 
rendered evident by its oxidation at the surface. 
Talcose slate is chiefly composed of tale and quartz, and sometimes of tale and feldspar. In 
texture it often resembles clay slate, but can be distinguished from that rock by the peculiar soapy 
touch of tale. When mica is present in this rock it receives the name of talco-micaceous slate. 
Chiorite slate—Chlorite often takes the place of mica and talc, and forms this rock. Like horn- 
blende slate, it is green, but it is much less hard. It occurs in this State but rarely. 
Clay slate, as its name implies, contains a large portion of argillaceous matter. In texture it is 
quite fissile. Its colors are various, and although sometimes dull, it often has a shining lustre. In 
the Districts bordering the tertiary series of South Carolina, this rock is well known for the abun- 
dance of excellent whetStones which it furniShes. The clay slate of the metamorphic series passes 
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