"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.
"These are—
- "(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.
GEOLOGY OF MOUNT ROYAL
"Mount Royal consists of a body of intrusive plutonic rock penetrating the nearly horizontal limestone of the Trenton formation (Ordovician). It consists of two main intrusions composed of essexite and nepheline syenite respectively, of which the nepheline syenite is the later followed by a swarm of dykes and sheets of consanguineous rocks which cut not only the main intrusions, but also penetrate the surrounding limestones in all directions. The intrusive rock in some places tilts up the limestones while elsewhere about the mountain these maintain their horizontal attitude. The intrusion may be essentially laccolitic in character, or it may represent the plutonic basis of a volcano. The erosion has been so long continued that it has been impossible as yet to reach a definite conclusion on this point.
"The greater part of the plain through which the mountain rises, and which is underlaid by Ordovician strata, is mantled by drift which also covers the slopes of the mountain. This drift, and in some places the underlying rock, has been terraced by a series of well defined beaches, which mark the successive stages of the retreat of the sea at the close of the Glacial age.
"The City of Montreal is built upon these drift deposits, and lies upon the slopes of Mount Royal and upon the plain about its foot. The development of the city was largely influenced by the position of the main beaches above mentioned.
"At a number of places on the slopes of Mount Royal and in its vicinity there are remarkable developments of igneous breccia. This has as a matrix one or other of the dyke rocks of the series, while the included fragments consist in part of the Trenton limestone, often associated with fragments of the other underlying stratified rocks traversed by the dykes in their upward passage. These fragments are frequently so numerous that they constitute a large part of the whole mass. Perhaps the most remarkable of these breccias is that which occurs on St. Helen's Island in the harbor of Montreal, and which is unique among these occurrences in that it contains fragments of rocks which are more recent in age than any of the sedimentary strata now found in the district.