OF SOUTH CAROLINA. 99 
commences the erystals of feldspar lose their edges, the cohesion of the rock is destroyed, it crum- 
bles to pieces, presenting in its debris the grains of quartz, feldspar and mica; and if it be at all 
elevated, they are washed down by every torrent. 
Sulphuret of iron is also a very common and efficient cause of disintegration. It is composed of 
sulphur and iron: the sulphur combines with the oxygen of the atmosphere, is converted into 
sulphuric acid, which unites with a portion of the iron, and produces a soluble salt, which is 
washed away, leaving a rusty streak of oxide of iron; and disintegration, to a certain extent, 
follows. But the cause of the great depth to which this disintegration extends is not equally 
obvious. Itis not uncommon to find wells sunk in granite so soft as to require for their excavation 
no other instrument than a spade. Near Abbeville C. H. a ravine has been washed by the rains, 
toa depth of thirty or forty feet. In the N. E. corner of Lexington; on Indian Creek, in Newberry; 
above Union C. H. on the road to Meansville; near Montecello and Winnsboro’, in Fairfield, simi- 
lar excavations may be seen. Sometimes it would be difficult to determine that the mass was in 
place, and not a heap of transported materials, but for the existence of seams and quartz veins, 
which may always be traced wherever the section presents a fresh surface. When the contem- 
plated rail-roads are commenced in the upper Districts this soft state of the rocks will be fully 
appreciated; for while the mass is easily excavated, it makes excellent embankments. 
It will be found that rocks are subject to this sort of ruin in proportion as they contain feldspar or 
hornblende; but there are other causes besides those just mentioned that are producing the same 
effect: among these may be mentioned the percolation of water between the upturned edges of the 
slates; the freezing in winter, and consequent expansion of the water, must have a constant tendency 
to break down and pulverize such rocks. 
The changes that occur in the decomposition of the rocks that contain much hornblende, are, if 
possible, still more remarkable. I saw an earthy mass at the base of Ceesar’s Head, that I took, at 
first sight, for red and yellow ochre, and it required some hard digging before I could convince my- 
self that it was nothing but decomposed hornblende slate. 
The trap rocks of the State afford curious examples of this sort, to. which I have already alluded. 
If the spheroidal pieces of trap scattered over the surface be examined, it will often be seen 
that they have a tendency to exfoliate, and if struck with the hammer they sometimes part in con- 
centric layers. 'The lustre is quite dull, and frequently the mass is so soft as to be impressed with 
a knife. If the decomposition takes place below the surface the next step is the production of a 
yellowish tenacious substance, like wax, but it is not plastic, like ordinary clay—it is this that 
forms the sub-soil in all the trap regions of the State—and the next is the conversion of this into 
a warm, chocolate-colored earth. ‘These changes can be studied in nearly every District in the 
upper country. : | 
Where rocks undergo such alterations of structure as these, over such wide areas, and to so 
great a depth, it is easy to see that the configuration of the surface must be constantly subject to 
change; and there are few persons residing in Edgefield or Abbeville, or in the parallel Districts 
across the State, that have not bad painful experience of the wasting effects of rains. It often 
requires all the observer's knowledge of the destructive and transporting force of water, fully to” 
appreciate the effects produced by a single Southern torrent of rain upon rocks in the condition 
just described. 
