"The Monteregian Hills are a series of ancient plutonic intrusions. Some of them (e. g. Brome Mountain) are apparently denuded laccoliths, one of them (Mount Johnson) is a typical neck or pipe, and it is probable that some, if not all, of them, represent the substructures of volcanoes which at one time were in active eruption in this region.
"It is impossible to determine accurately the date of these intrusions. In the case of Mount Royal, however, inclusions of Lower Devonian limestone are found in the intruded rock, so that the intrusions forming the mountain are later than Lower Devonian time.
"Since Dresser by another line of evidence, has shown that the intrusion of Mount Shefford probably took place before late Carboniferous time, the Monteregian intrusions probably date back to the late Devonian or early Carboniferous period.
"It must be noted that while six of these mountains rise from the horizontal strata of the plain, the two most easterly members of the group, namely Shefford and Brome, while still to the west of the axis of that range, lie well within the folded belt of the Appalachians, although, owing to the extensive denudation from which the region has suffered, this folding has had but little influence on the local topography. About La Trappe, at the extreme westerly extension of the Monteregian area, the dykes of the series cut rocks of Laurentian age, which here form an outlier of the great Laurentian protaxis on the north.
"The Monteregian Hills form an exceptionally distinct and well marked petrographical province, being composed of consanguineous rocks of very interesting and rather unusual type. These are characterized by a high content of alkali and in the main intrusion of almost every mountain two distinct types are found associated with one another, representing the products of the differentiation of the original magma.
"(a) Nepheline syenite, in some cases replaced by or associated with pulaskite, tawite, akerite or nordmarkite.
"(b) Essexite, in some cases represented by theralite, yamaskite, rougemonite, or rouvillite.
"It may be mentioned that yamaskite is a very basic rock type characterized by a great predominance of pyroxene, basaltic hornblende and ilmenite, with about two per cent of anorthite. Rougemonite consists largely of anorthite with pyroxene as the only important ferro-magnesian constituent. Rouvillite is a highly feldspathic variety of theralite.