16 GEOLOGICAL SURVEY 
the side of the obtuse angle e, made by the vein and cross course ; and this, it is plain, will hold 
good, no matter which end of the vein we are pursuing. 
Mining, in our gold regions, has not been pursued with sufficient regu- 
larity to enable us to ascertain whether or not this law holds good in them, 
and the subject is introduced for the purpose of directing the attention of 
our miners to a fact so highly important. 
The origin of the metallic matter in veins has excited a good deal of 
speculation, but it still remains in obscurity. Four theories have been 
proposed to acconnt for it. The Ist is that proposed by Werner, who sup- 
posed that veins were filled from above, by aqueous solution. The 2d is 
the theory of Hutton, that proceeds upon the supposition that veins were 
injected from below, in a state of igneous fusion. The 3d accounts for the 
repletion of veins by the process of sublimation from intensely heated mineral matter, by which 
cracks and fissures in the superincumbent rocks were filled. And the 4th theory supposes veins to 
be the result of slow segregation of matter from the adjacent rocks, either into cavities formed 
during or subsequent to their deposition, or into fissures which may be caused by fracture or dislo- 
cation of the consolidated rocks. This segregation may result from electro-chemical action, as 
illustrated by the experiments of Fox, Becquerel, and, more recently, by those of Hunt. 
Much might be said in favor of each of these hypotheses, and it is highly probable that metal- 
liferous veins are not the result of either.exclusively, but may be due sometimes to one, and again 
to the others. It is quite certain that where pebbles and other surface materials are found in veins, 
they must have been filled from above ; and when we find the mineral matter in a vein embedding 
fragments of the walls torn off in the passage of the mineral matter through the rocks, it is equally 
certain that such veins are produced by injection from below. Numerous cases occur where metal- 
lic matter is enclosed on all sides in the rocks, and the origin of which must be referred to segre- 
gation. 
The richness of veins, where the rocks are much disturbed, as well as where granite comes in 
contact with the overlying stratified rocks, is well known; and it is quite probable that this richness 
is due to the rents and fissures that intersect the rocks at these points, which furnish favorable recep- 
tacles for ores and metals. 
It would be desirable, if it were possible, to present rules for determining the presence of useful 
ores and minerals; but the only reliable indications are those derived from a knowledge of the 
geological structure of the country, added to a careful study of the recorded results of experience 
and observation. Many useful minerals are peculiar to certain geological formations. No one 
acquainted with its geology, would think, for example, to look for coal in the rocks of South 
Carolina; nor would he expect to find any other metal than iron in the lower part of the State—at 
least in workable quantities. Coal is of vegetable origin, and of course cannot occur in rocks that 
preceded the existence of plants on the earth, and iron is the only ore that is found in all the form- 
ations in sufficient quantity to be worked. 
Gold, in the United States, is principally confined to the slates of the metamorphic rocks—gene- 
rally the taleose and talco-micaceous slates ; but in South Carolina it also occurs in granite, gneiss, 
and hornblende rock. The gangue is universally cellular, ferruginous quartz, and iron pyrites, 
