Jur.Y, 1889 . 
A GRANITE CONTAINING LITHIA. 
159 
The occurrence of tourmaline is very frequent among the 
granites of Devon and Cornwall, and topaz occurs occasionally, 
as at St. Michael’s Mount, and both minerals are constantly 
associated with tin stone. The presence of fluorine in these 
minerals has suggested to some geologists that they are 
secondary formations, and that it is possible that the gaseous 
agent in the production of them was the fluoride of tin 
which is easily volatilised, and is also easily decomposed by 
the action of water affording oxide of tin for the tin stone, 
and hvdrofluoric acid, which would be able to effect further 
changes in the rocks. Fluorine is also a constituent of 
lepidolite, so that the association of minerals is quite a 
natural one. 
The presence of lepidolite among the constituents of the 
Cornish granites was mentioned by Mr. Allport, in his paper 
on the rocks surrounding the granite masses of the Land’s 
End (Q. J. Gr. S., 1876), and I have mentioned to this Section 
the very widespread diffusion of lithia among granitic micas 
from all parts of the kingdom, but I have not come across 
any other except one Scotch muscovite which contained 
sufficient to at once colour a flame so as to be visible 
without a spectroscope. 
In this connection it is interesting to recall the occurrence 
of a very large percentage of lithium chloride, viz., 26 grains 
to the gallon, in a spring in Clifford United Mines, in 
Cornwall, as mentioned by Dr. Miller. 
It is obvious that such a rock as we have before us this 
evening, on being subjected to those influences which result 
in the kaolinising of the felspars, and the alteration at the 
same time of the mica, would be very likely to furnish a 
spring water with a large amount of lithia. 
The spectroscopic means at my command did not enable 
me to examine the mica for the still rarer alkalies, rubidia and 
caesia. They occur in some of the lepidolites of Maine in 
rather considerable quantity; indeed, I believe, the quantity 
contained in some (2-44% of rubidia, and 072% of caesia) is 
larger than in any other known substance, except the 
mineral pollux, which contains 32 % of caesia. A thorough 
examination of the alkalies on a large scale would be of great 
interest. 
It will be obvious that this paper is very imperfect, and it 
seems w 7 orth while to recall the opinion of Professor Judd, 
that the microscopical study of rocks, even when combined 
with the chemical study of them, should be subordinated to 
the geological or field examination, and not take the place 
of it. In this case it is plain that the question as to the 
