158 PROCEEDINGS OF THE BOSTON MEETING 



copper carbonate. The sulphide minerals are chiefly chalocite and covellite, 

 with traces of bornite. 



The copper is believed to have been derived from the greenstone by warmed 

 circulating waters, perhaps of meteoric origin. The chalcocite was, with the 

 exception of a little covellite, the latest sulphide to be deposited. It is con- 

 sidered to have been formed from the warm solutions and to be primary in 

 origin. 



At a later period oxidizing surface penetrated the ore bodies beyond the 

 lowest workings, a maximum distance of 1,500 feet beneath the surface, and 

 converted part of the copper sulphides into copper carbonate. The oxidation 

 of the sulphide was pervasive and feeble ; never absent, yet nowhere complete. 



The advent of the Glacial Period arrested the deep oxidation by the freezing 

 of the oxidizing waters, and they have remained frozen until this day. Thus, 

 the oxidation was preglacial and the present ground water is a frozen one. 



Postglacial oxidation has been negligible, but mechanical disintegration of 

 the ore cropping during this period has been sufficiently vigorous to produce a 

 sulphide talus-slope ore body and an ore body of broken sulphide debris in a 

 glacier, which is now about to be mined. 



Presented in abstract from notes. 



CO^^DITIONS AT VESUVIUS IN 1910 

 BY HENRY S. WASHINGTON 



Presented without manuscript. 



SWEET GRASS HILLS, MONTANA 

 BY JAMES F. KEMP AND PAUL BILLINGSLY 



(Abstract) 



The Sweet Grass Hills are the most northerly of the laccolithic mountain 

 groups lying east of the Rocky Mountains in Montana. They are the only 

 remaining group whose petrography has not been worked up in considerable 

 detail. They were the object of a hasty visit by Dr. George M. Dawson in the 

 seventies, when the region was a dangerous Indian country. Specimens then 

 collected were determined shortly after by F. D. Adams, and fifteen years or 

 more later three specimens of special interest were described by W. H. Weed 

 and L. V. Pirsson. The present writers spent a week together in the hills in 

 May, 1918, reviewing earlier work by the junior writer, and have been aided 

 by further details from George M. Fowler, a geologist, resident for a year in 

 the region, in charge of oil exploration. The petrographic details have been 

 subsequently worked out by the senior writer. 



The three separate and striking groups of laccoliths constituting the three 

 buttes are described and illustrated and their relations to the sedimentary 

 rocks are shown. Excellent sections of the sedimentaries to the depth of 2,000 

 feet have been afforded by four wells sunk for oil. The laccolithic centers are 

 surrounded by sills and dikes in great numbers. 



There is notable range in variety in the laccoliths, but the rocks are, on the 



