252 FRANK D. ADAMS 



sions in Yamaska, Rougemont, and Montarville have been deter- 

 mined. 



With regard to the structure of these mountains, it may be 

 noted that Logan, who first examined them, refers to them as 

 "intrusive masses breaking through the surrounding Paleozoic 

 strata."^ They are thus represented in the geological sections 

 of this district contained in the atlas accompanying his report. 

 Ells refers to them simply as "eruptive mountains."^ The more 

 detailed studies of Shefford and Brome mountains recently 

 carried out by Dresser, however, have led him to consider these 

 two occurrences as uncovered laccolites. Concerning Shefford 

 mountain he says : 



The sedimentary strata which surround the mountain .... are found 

 to wrap around the igneous mass of the mountain, mantling it with a 

 hardened contact zone to a height of 300 to 1,000 feet above the surrounding 

 country, according to the direction of glaciation. Above the latter height 

 the mountain rises upward of 200 feet, the summit being capped by an 

 outlier of Trenton slate about a quarter of a mile in extent. This preserves 

 the cleavage, dip, and strike of the similar rock at either side of the moun- 

 tain and is penetrated by dykes from the underlying igneous rocks. From 

 these facts, together with the absence of tufaceous material and the general 

 arching of the strata around the mountain, it is inferred that Shefford moun- 

 tain is an uncovered laccolite rather than the denuded neck of a once active 

 volcano. ' 



In Brome mountain also the presence of outlying masses of the 

 surrounding sedimentary series at high levels lying upon the 

 igneous rock of the intrusion " seem to indicate unmistakably 

 that Brome mountain, like Shefford, is an uncovered laccolite 

 and has never been an active volcano." '^ 



Mount Johnson, on the contrary, as will be shown, is a typical 

 neck or plug, representing a portion of the conduit through 

 which the magma rose, to fill laccolites above in strata which 

 have long since been swept away by erosion, or to be poured 

 out at the surface at volcanic vents. This is seen by the fact 



"^ Geology of Canada, p. 655. 



^Ann. Rept. of Geol. Surv. of Canada, Vol. VII, J, p. 71. 



^American Geologist, October, 1901, p. 204. 



* Geo/. Stirv. of Canada, Su?nmary Rept. for i go i, p. 187. 



