156 
A GRANITE CONTAINING LITHIA. 
July, 1889. 
The comparative quantities of potash and soda will also 
be noticed ; even if the lithia be regarded as replacing potash, 
there is not such a preponderance as is usual in granites. 
The rock is mentioned by Mr. Teall, in his “ British 
Petrography.” He says (p. 316): “ A remarkable variety of 
granite occurs at Meldon, near Okehampton. It is almost a 
pure white, Somewhat resembling statuary marble in appear¬ 
ance, and is composed essentially of quartz, felspar (largely 
plagioclase), white mica, and topaz. Black mica is absent; 
the rock is therefore a Muscovite granite. It contains also green 
tourmaline.” In a note he speaks of the topaz as occurring in 
grains which but rarely show good form in thin sections, but 
gives reasons which make the identification certain. 
The constituents of the specimens which I have had the 
opportunity of examining are, on the whole, of medium size. 
The dimensions of the separate grains of quartz and felspar, 
are about the same in all directions, and on an average 
from 2 V f° tV of an inch across—some of the felspars being, 
however, somewhat elongated. Occasional veins of larger 
crystals traverse the mass, in which the aggregation of the 
various minerals is much looser than in the more close 
grained parts, so that it has not been possible to make 
sections. As the constituents do not seem to vary in kind, 
this is not of so much importance, and the easy separation 
of the mica makes it a simple matter to determine the fact 
that the lithia occurs in this mineral and not in the felspar. 
I 11 these veins, too, the tourmaline occurs in fairly large 
crystals of a beautiful pale-green colour, or sometimes of a 
very delicate pink. The two colours are often present in the 
same crystal, the one forming a core, the other surrounding 
it. The crystals were of sufficient size to permit of the 
determination of the angle over the edges of the terminal 
rhomb by means of the reflecting goniometer, and fragments 
give the blowpipe reaction for boric acid with the utmost 
distinctness. On examination of a thin slice, we find that 
almost all the felspar is of one of the triclinic varieties. In 
such sections as show a symmetrical extinction with regard to 
the plane of twin composition, the extinction angle is 
generally low. I have seen no case in which the presence of 
labradorite or any more basic felspar could be determined, 
and indeed the very small amount of lime, as found by 
analysis (0'9 o /o), makes the presence of such impossible. 
The question naturally arises as to how the alkalies are dis¬ 
tributed, whether both in one felspar or in two separate felspars, 
such as orthoclase and albite; or, finally, whether most of the 
potash is present in the mica. The very great preponderance 
