66 GEOLOGICAL SURVEY 
penetrates the rocks through which the dykes pass may be observed at many places, in ravines and 
such localities. On the surface the dykes are readily traced by the long lines of black or dark 
green, and ferruginous, spheroidal masses of trap, scattered along in the direction of the dykes, 
often for miles in extent. The direction of these dykes is exceedingly uniform, varying between 
15° and 35° East of North, and in general they are but slightly inclined from vertical. 
I have no where observed the very decided columnar structure characteristic of these rocks, but 
they often occur sufficiently prismatic to present the idea of an artificial wall, and this impression 
is strengthened when the stones forming the dyke remain, as they sometimes do, piled up a few feet 
above the surface, while the surrounding rocks are washed away. 
A remarkable wall of this description was pointed out to me by Mr. Nesbit, at te Mountain 
shoals of Enoree, where the straight lines of the faces of the dyke, together with the “ breaking of 
joints,” presented by the four-sided prisms which are laid across the wall, very forcibly suggests 
the remains of some ancient military work. 
At one of the mines in Kershaw District a dyke, about twelve feet thick, completely divides the 
auriferous rocks, which have been worked on each side of it, to the depth of thirty feet, leaving the 
dyke standing undisturbed. Between the dyke and walls of talcose slate there is about two feet of 
yellowish green wacke, and some of the fissures are lined with crystals of sulphate of alumina. 
The slates at the point of contact with the dyke 
are blackened and otherwise much altered, a cir- 
cumstance not common in the talcose slates,— 
Fig. 12 represents this dyke. 
Wacke, which seems to be decomposed trap, is 
not unfrequently associated with the dykes of the 
State. It is sometimes white, like chalk, may be 
cut with a knife, and is often mistaken for soap- 
— 2 2— , having jointed structure. 0 ‘ 
meee eee = stone. Such is the case at a locality on Hard- 
labor Creek, where a reddish variety has been cut 
for hearth-stones, under this impression. 
Ezell’s mine, in Lancaster, is intersected by a dyke, which, at the surface, and for some feet 
below, is nothing but red clay. In expioring the mine, it offered a tempting site for a shaft, but it 
had not been prosecuted far before the trap assumed its true character, and put an end to further 
operations in that direction. 
The dykes of South Carolina vary in thickness from a few inches to several miles: in the latter 
case, however, it is probable that the rock is spread out over the surface by overflowing from the 
fissure throug which it was ejected from below, in a molten state. 
It will be unnecessary to describe all the dykes of the State, particularly as they present but few 
peculiarities, and a description of one will apply, with certain limitations, to the whole. TI have 
indicated, on the map, nearly all those that I have examined; but many more must have escaped 
me. A glance at the map will give a very correct idea of their relative position and distribution. 
The smaller dykes are of course not conspicuous, and would pass unnoticed by any one but a 
Geologist. In those Districts, however, where the trap rocks abound, they have impressed them- 
selves so distinctly on the soil and physical features of the country, as to suggest particular, but by 
