OF SOUTH CAROLINA. 3 
with difficulty to the hammer—a fact known to the Aborigines of the country, who used them as 
the principal material in the construction of their tomahawks and other instruments requiring much 
strength. 
Argillite, which is rather a rock than a simple mineral, is well known on account of its exten- 
sive use as common writing slate. It has a fine texture, an earthy fracture, and presents many 
shades of color, such as black, grey, red and purple. Alumina, its principal ingredient, next to 
silica, is the most widely diffused substance. 
Limestone is best known in the numerous varieties of marble. It is sometimes crystalized, and is 
then called calcite, or cale-spar. It is softer than feldspar, and may be readily scratched. It is easily 
distinguished by the effervescence produced by the application of an acid. It is more abundant in 
the newer rocks, and constitutes about one-seventh of the rocks of our globe. 
If attention be now directed to the mineral masses or rocks made up of these simple minerals, 
and which constitute the earth’s crust, so far as man has penetrated, it will be seen that they are 
disposed in beds or layers, piled wp, one upon another, in the manner in which matter would be 
deposited which was once held in suspension in water; or else they occur in masses of uniform 
structure and appearance, and without any traces of those beds or planes that would indicate a 
mechanical origin. And thus we have a very natural division of rocks, into two great classes— 
Stratified and Unstratified rocks. 
UNSTRATIFIED Rocks. 
The examination of this class of rocks will not proceed far before it is seen that it is composed 
of two groups, varying widely in structure and external appearance. The one consisting of crys- 
taline rocks, in which each of the component minerals presents the appearance of crystalization, 
independent of the mass. ‘They underlie the stratified rocks, or are pushed up through them, and 
may, therefore, be regarded as the foundation, or frame-work, of the earth’s crust. The rocks 
composing the other group occupy a less important place, generally only filling the cracks and 
fissures in other rocks, or spread out over their surface, where they frequently assume regular forms. 
These two groups or classes of rocks may be designated as the granitic and the basaltic rocks. 
Granitic Rocks. 
Common granite, which is an aggregate of quartz, feldspar and mica, is the type of this class of 
rocks. It is either coarse or fine, generally depending upon the quantity of the feldspar. Its colors, 
which are also much influenced by that mineral, are white, gray, or red. Its component minerals 
appear aggregated promiscuously, and sometimes as if the mica and feldspar were embedded in a 
quartz base. The ease with which granite splits in two directions, when quarried by a skilful 
workman, is due to the coincidence of the cleavage planes of the felspar with those of the blocks 
split out. Fractures or seams of considerable extent, are often seen in granite—not unfrequently 
dividing it into cubical blocks ; but these are the effects of a different cause. 
Granite has various names, derived from the different proportions and mode of aggregation of its 
constituent minerals; it is called micaceous, feldspathic, or quartzose, as mica, feldspar, or quartz 
may predominate, 
Porphyritic granite has the feldspar disseminated through the whole in large crystals, which are 
