212 Canadian Record of Science. 



them as " intrusive masses breaking through the surround- 

 ing Paleozoic strata ' n They are thus represented in the 

 geological sections of this district contained in the atlas 

 accompanying this report. Ells refers to them simply as 

 " eruptive mountains." 2 The more detailed studies of Shef- 

 ford 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 mountain 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 mountain is an uncov- 

 ered laccolite rather than the denuded neck of a once active volcano. 3 



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." 4 



i 



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 that the flat-lying strata all about it 

 are not arched up, but abut sharply against the igneous 

 core of the mountain and are cut off' by it. Being shales, 

 they are of course baked to hornstones, but show no signs 



1 Geology of Canada, p. 655. 



2 Ann. Rept. Gel. Surv. of Canada, Vol. VII, J., p. 71. 



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



4 Geol. Surv. of Canada, Summary Rept. for 1901, p. 187. 



