350 Dresser — Geology of Brome Mountain. 



This age limit virtually agrees with that of the adjacent 

 intrusion of Shefford mountain which shows similar dynamic 

 metamorphism, but cuts slightly later strata, viz.. The Farn- 

 ham black slates (D3a), a division of the lower Trenton. The 

 latter, however, do not occur at Brome. 



The presence of numerous sedimentary outliers which have 

 been invaded and otherwise altered by the igneous rocks 

 beneath, together with the general character of those rocks, 

 and the absence of any tufaceous material, seems to indicate 

 that Brome mountain is an uncovered laccolite and has never 

 been an active volcano. The evidences of a similar structure at 

 Shefford mountain are most conclusive, and as the distance 

 between the mountains is only two and a half miles, which is 

 less than the smallest diameter of either, they are probably 

 parts of a single laccolite. The correspondence of the rocks in 

 the two masses also favors this view. 



Petrography. 



The igneous rocks of which Brome mountain is essentially 

 composed belong to three principal types, two of which cer- 

 tainly are the products of separate irruptions, and the third 

 possibly so. The rock of the earliest intrusion is of the Essexite 

 family, according to the Rosenbusch classification, or in the 

 Quantitative Classification* it is a Hessose. 



The second is of a syenitic character ranging from Nord- 

 markite" to Nepheline syenite. It is Nordmarkose in the 

 Quantitative Classification. The third, which is of compara- 

 tively small extent, is a porphyritic rock which from its micro- 

 scopic and chemical character is classed as a phyro-laurdalose. 

 Their distribution is shown on the accompanying figure (2). 



The nordmarkose distinctly cuts the hessose, but the contact 

 of the laurdalose with the nordmarkose, which completely 

 surrounds it, is everywhere drift-covered so that conclusive evi- 

 dence of their relations could not be obtained. 



Hessose. — This is a massive rock, gray in color, and weather- 

 ing to a dull brown. Its structure is granitoid and the texture 

 medium. Feldspar and small amounts of dark minerals, chiefly 

 hornblende, mica, and iron ore, can be seen by the unaided eye. 



In the thin section feldspar is found to constitute fully 90 

 per cent of the rock, in parts that are considered typical, the 

 remaining constituents being pyroxene, olivine, and biotite 

 with accessory magnetite and apatite. Hornblende in many 

 cases occurs quite as abundantly as pyroxene, but in other parts 

 of the rock is entirely wanting. The structure in general is 

 hypidiomorphic granular. 



*" Quantitative Classification of Igneous Rocks," Cross, Iddings, Pirsson 

 and Washington, The University of Chicago Press, 1903. 



