Dresser — Geology of Brome Mountain. 349 



In 1901 the present writer completed a report* .on Shefford 

 Mountain which. is now in press, and published a resume' of it 

 in the American Geologist for October of that year (vol. xxviii). 



The above mentioned investigations have shown that the 

 Monteregian Hills are of igneous origin and are intrusive in 

 their relations to the strata surrounding them. As a petro- 

 graphical province they are distinguished by two main rock 

 types, one representing a basic magma of the essexite class in the 

 Rosen busch classification — the other, various types of alkali- 

 syenite. In structure Mount Johnson, and probably Mount 

 Royal, are true volcanic necks, while Shefford has been found 

 to be a laccolite. 



It is the purpose of the present paper to outline the main 

 features of Brome Mountain, and to indicate its general rela- 

 tion to the other hills of the Monteregian series that have been 

 thus far studied. 



Brome Mountain is the largest hill of this series. It com- 

 prises an area of about thirty square miles in the counties of 

 Brome and Shefford. Together with Shefford Mountain, 

 w T hich stands two and a half miles to the north, and is next to 

 it in size, Brome is the most easterly of the Monteregian Hills. 

 In form it is rudely circular. The central portion, about 

 Brome pond, is a nearly level basin two and a half by one and 

 a half miles in extent, and overlain by heavy beds of post-gla- 

 cial clay. The interior basin has an average altitude of about 

 five hundred feet above sea level, or only a little above the 

 country surrounding the mountains ; w T hile the basin is encircled 

 by a rim of hills which rise to heights of from six hundred to 

 one thousand feet above the surrounding plain, or one thousand 

 to fifteen hundred feet above the sea. "Pine Mountain" is 

 the highest point. 



In common with the other hills of the Monteregian series, 

 Brome is an igneous mass intrusive through Paleozoic strata. 

 The latter belong to the Sillery division of the Cambrian system 

 on the north, east, and south sides of Brome mountain, and on 

 the west to the Mystic series (D2&) of the upper Chazyf. 



The latest time at which the intrusion could have taken 

 place is also indicated, though less definitely, by the fact that 

 the igneous rocks are somewhat foliated, and show r in places an 

 incipient schistose structure. This is parallel in direction with 

 the schistosity of the surrounding sediments, though much less 

 in degree, and represents a late stage in the folding of the 

 Appalachian uplift. As this was not shared in by the Permo- 

 Carboniferous of the maritime provinces, Brome mountain was 

 formed after the deposition of the upper Chazy sediments and 

 before the close of the Carboniferous period. 



* Geological Survey of Canada, vol. xiii, part L. 



\ Dr. R. W. Ells, Annual Report of the Geological Survey of Canada, 1894. 



Am. Jour. Sci.— Fourth Series, Vol. XVII, No. 101.— May, 1904. 

 24 



