APPENDIX. 
107 
lite dikes. I regard these granite dikes as of the same age of the Chrys- 
olite. They are both due, no doubt, to the same disturbing force. This 
seems evident from the chrystaline character of the Gneiss enclosing 
them. The granite dike matter is often well defined, and very distinct 
from the enclosing wall rock. I observed in the Clarissa Buckhanon 
mine, in Mitchell, — a mine operated by Capt. J. K. Irby, at the time of 
my visit in the summer of 1873 — that the walling remained, very often 
unbroken, when the vein matter was blasted out. That mine, when I 
saw it, was a fine example of a dike fissure filled with a coarse granite. 
Indeed, true mica mines belong to these granite dikes. I have visited 
quite a number of workings all along the belt from Mitchell to Georgia, 
and have not yet seen a good productive mine of mica, only where the 
evidence was clear as to the dike character of the vein. Nor do these 
well defined veins invariably make productive mines, for the reason that 
some fissures are filled with barren matter, while in others the law of 
crystalization acted on a much larger scale in one than in another. It is 
worthy of special note, that where, for example, the hexagonal faces of 
the mica crystals range from one to three inches, this law as to size holds 
good in that vein. Again, when the prospecter finds mica in his vein, 
having what the miners call a straight edge, which is only one face of a 
crystal of mica, say six inches in length, though the remainder of the 
crystal may be rough and irregular, he may be pretty certain that the 
vein will yield blocks of good size, and the only remaining question with 
him is as to quantity and quality. Some dikes seem to have been filled 
with something like equal proportions of Feldspar, Quarts and Mica, and 
it sometimes so happens that a large per cent, of the mica in a vein has 
been very imperfectly crystalized, the plates interlocking, making a sort 
of gnarled mass, rather than regular transparent plates. These facts 
should be borne in mind by prospecters for mica mines. A good deal of 
money has been spent upon large seams of Feldspar, which occasionally 
occur in Gneiss rock. Some times a few blocks or crystals of glass are 
obtained of respectable 6ize from these seams. But they being a part of, 
and incorporated with, the other elements of the Gneiss, are unreliable 
and in no instance have made valuable mines. Another thing to be con- 
sidered is the lithological character of the rocks in which the dike veins 
occur. At some localities the walling is altogether Gneiss, at others one 
wall is Gneiss and the other Syenite; and again, I have seen the walling 
a sort of Chlorite shale having Hornblend in its composition. My obser- 
vations have brought to light an important fact in connection with these 
respective wallings to regular Mica veins. "Wherever one of the walls 
is Hornblendic rock the Mica is more liable te be specked by Magnetite 
