204 ^/^^ American Geologist. October, 1901. 
rises nearly twelve hundred feet above the surrounding coun- 
try or about sixteen hundred feet above mean sea level. 
With the larger and adjacent mass of Brome mountain, 
which work in progress shows to belong to the same series, 
Shefford mountain lies well within the folded belt of the Ap- 
palachians, though owing to the extensive denudation from 
which the region has suffered, this fact hsfs little if any dis- 
cernible influence on the topography of the immediate local- 
ity. The sedimentary strata which surround the mountain 
have been shown tO' be of Sillery (Lower Cambrian) and Low- 
er Trenton age. They are found to wrap around the igneous 
mass of the mountain mantling it with a hardened contact 
zone to a liight of three hundred to one thousand feet above 
the surrounding country, according to the direction of glacia- 
tion. Above the latter hight the mountain rises upwards of 
two hundred feet, the summit being capped by an outlier of 
Trenton slate about a quarter of a square 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 tuffaceous 
material, and the general arching- of the strata around the 
mountain, it is inferred that Shefford mountain is an uncov- 
ered laccolith rather than the denuded neck of a once active 
volcano. It is also evident that the depth attained by erosion 
in this part of the St. Lawrence valley cannot have been less 
than twelve hundred feet, and may have been much more. Its 
maximum does not seem to admit of any very reliable means 
of computation. The restoration of the folds of the strata, 
which are altogether worn down, would be the natural method 
of investigation, but the strike of cleavage, coinciding with 
that of the bedding, while the dip of the two is probably very 
different, so far complicates the question as to seemingly 
preclude the possibility of any Init the most hypothetical cal- 
culations. 
In the igneous mass three classes of rocks whose field 
relations show them to be products of as many separate per- 
iods of irruption (map) are easily distinguished. They are in 
order of intrusion, essexyte, nordmarkyte, and pulaskyte. 
Their comparative mineralogical composition may be shown 
as follows : 
