OF SOUTH CAROLINA. 5 
and the consequent contraction, the fractures probably taking the direction of the cleavage planes of 
some of the constituent minerals. 
These columns often present a jointed structure, the result of cleavage parallel with the base, 
which is doubtless due to the same cause. 
Fingal’s Cave and the Giant’s Causeway are well known and magnificent examples of columnar 
basalt. Numerous localities of trap occur in the upper districts of South Carolina. 
Greenstone is composed of feldspar and hornblende. It is granular, and sometimes the crystals 
of its component minerals may be distinguished. 'The hornblende generally predominates, and im- 
parts its characteristic green color to the mass. 
Amygdaloid. ‘This name is applied to those varieties of trap or basalt having vesicular cavities 
filled with kernel shaped minerals. It is probable that the materials composing the minerals found 
access to these cavities by the percolation of the water holding them in solution. 'The absence of 
those silicions minerals which abound in the granitic rocks is a remarkable feature in the rocks of 
this class. 
Eurite is, for the most part, composed of compact feldspar: its color is white or grey. It occurs 
in South Carolina in the form of veins or dykes. 
Porphyry. This name was at first applied to a certain class of stones susceptible of a polish, 
and characterized by a red color. But it is now restricted to rocks having a compact base, generally 
of feldspar, hornstone or claystone, with crystals or grains of some other mineral scattered through 
the mass. These crystals or grains are very frequently common feldspar of various colors. 
Igneous rocks. The unstratified rocks thus briefly described are often designated by the name 
of igneous rocks. 
Speculations upon the form in which the materials composing our planet first came from the 
hands of the Creator, would be evidently beyond the limits of this introduction. Yet there is some- 
thing as beautiful as it is simple in the thought that these materials once existed in a gaseous state ; 
that condensation took place, and a consequent evolution of heat sufficient to fuse the solid substan- 
ces resulting from condensation ; that this molten mass, cooling down by slow radiation, produced 
the unstratified rocks. There is in this simple process so much in unison with the simplicity and 
unity of those causes in nature which are constantly producing the most stupendous effects, that 
the mind rests upon it with pleasure. 
Whether the solid materials of our globe existed originally in the gaseous state or not, we have 
abundant evidence that the state which pre-existed the consolidation of the unstratified rocks was 
that of fusion. For they have altered-the rocks through which they pass in a manner quite analo- 
gous to the alteration that would take place on contact with intensely heated matter. They have 
been imitated or reproduced artificially by fusion. Trap rocks can scarcely be distinguished from 
the molten products of modern volcanoes. Where there is any marked difference it is owing to the 
fact that trap rocks have cooled slowly under great pressure, whilst the lavas of modern volcanoes 
have been ejected in the open air. The increase of temperature which is observed as we descend 
below the suiface, and which amounts to 1° Far. for every 45 feet of depth, as well as the exist- 
ence of thermal springs, seems to point to a source of heat beneath the earth’s surface, even at the 
present day. 
It is impossible to examine the relation of trap rocks to the sedimentary deposits without coming 
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