62 



GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



The following letter is also published by the kind ])ermission of Mr. 



Flynt: 



Massachusetts Institute of Technology, 



February 14, 1881. 

 Messrs. W. N. Flynt & Co. 



Gentlejcen : The two sorts of Modsou granite which were sent to us have beeii 

 carefully averaged, aud several analyses of each have been made, with the following 

 results : 



Silica 



Alumina 



Magnetic oxide of iron 



Lime 



Magnesia 



Soda 



Potash 



Sulphur Trace. 



Copper • Trace. 



Light, 



73.47 



15.07 



1.15 



4.48 



.12 



5.59 



.38 



100.26 



69.35 

 18.83 

 2.00 

 5.94 



!■ 3.78 



In this granite-mica is replaced by hornblende,' as in the Quincy and Rockport 

 granites. Such stones are much less aifected by chemical agents than those which 

 contain mica. 



The percentage of the alkalies, potash and soda, has much to do with the power 

 of resisting atmospheric influences. The amount of alkali in both specimens is 

 uncommonly small for granites. The sum is less in the dark-colored one, because 

 that contains a much larger proportion of the black hornblende, which probably is 

 free from alkali. The potash and soda come from the feldspathic ingredient of the 

 mixed minerals. This part seems to correspond nearly to andesite rather than to 

 ordinary potash-feldspar. 



We have in Hull's Treatise on Building aud Ornamental Stones the analyses 

 of thirteen European granites, which show, respectively, as the sum of the potash and 

 soda: (i.74, 7.27, (5.16, 7.91, 7.74, 7.98, 8.61, 9.67, 7.46, 5.G3, 7.40, 5.82, 9.00 per cent, 

 while the Monson granite shows 5.97 and 3.78. So far as the alkalies are concerned, 

 the dark is superior to all of the thirteen, and the light is better than all except the 

 twelfth, which was from Mont Blanc, aud the tenth, which was from Meineckenberg. 



The iron in the Monson granites is in the form of magnetic oxide, which is 

 unchangeable. Were it in the form of pyrites it would be liable to oxidize and would 

 give rusty stains on long exposure. 



' This is a mistake, as the black constituent of the specimens analyzed, which were submitted 

 to me for study, was biotite. The alkali determination given above differs greatly from that given 

 for the Becket gneiss and from the results of microscopical examination, which indicate the presence 

 of potash-feldspar. 



