"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.
"At the present time a tunnel, about three and a half miles in length, is being driven through Mount Royal by the Canadian Northern Railway, in order to gain an entrance from the westward to their proposed terminals in the vicinity of the corner of Dorchester and Ste. Monique streets, in the City of Montreal. It has afforded an excellent opportunity of studying the distribution of dykes, sheets, etc., as well as fresher specimens of many of the rock types of the district. Already about two miles and a half of the sub-heading have been driven. More minute description, in detail, of the various explored strata of rock is to be found in the same work."