Minerals in Gneiss. 



397 



while the exfoliated fragments would go to make up the brecciated 

 mass above described. 



After what I have said under graywacke and mica slate, in respect 

 to the origin of anthracite and plumbago, it will be repetitious to add 

 any thing farther on this subject. It may be that I am too much cap- 

 tivated with the beautiful simplicity of the principle, which refers 

 every variety of peat, lignite, bituminous and anasphaltic coal, and 

 plumbago, to a vegetable origin. But at present, I must regard that 

 opinion as far more probable than any other. 



It will be well for observers to bear in mind a remark of Dr. Davy 

 relating especially to the gneiss of Ceylon. "It is worthy of remark," 

 says he, " that graphite is generally found in company with gems. 

 I have had so often occasion to make the observation, that I now 

 never see the former without supposing the presence of the latter."* 



A mineral occurring in our gneiss, and often confounded with 

 graphite, is the sulphuret of molybdenum. I have noticed this in 

 Brimfield in scales ; and in the north part of Shutesbury, a little east 

 of Locke's Pond, it has been found in hexagonal plates nearly an 

 inch in diameter. 



The mineral found in our gneiss, which next to graphite will 

 excite most interest in an economical point of view T , is the native 

 alum mentioned in a postscript to the first part of my Report. It 

 is there stated to be found efflorescing on mica slate ; but I have sat- 

 isfied myself that it is that schistose variety of gneiss, which is pass- 

 ing into mica slate. There is mixed with it more or less of sul- 

 phate of iron, and both minerals proceed from the decomposition of 

 iron pyrites, and probably feldspar. This last mineral contains, as 

 is well known, a considerable quantity of potassa ; and I can imag- 

 ine no other source from whence this essential ingredient of alum 

 should be obtained. Nor will any one doubt, who has seen how 

 thorough is often the decomposition of the gneiss that contains pyri- 

 tes, that this potassa might be separated. I am not aware that alum 

 has been heretofore found in gneiss : but since this rock does contain 

 so much potassa, and if it can thus be separated from the feldspar, why 

 may not our gneiss prove a very prolific source of alum ? I do not 

 know that any special efforts have been made to ascertain whether it 

 can be procured in much quantity from the rock in Leominster : but 

 recently I have received a specimen from Barre, and it occurs also 



* Geological Transaction, vol. 5. p. 323. 



