34 
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 con- 
tinued that it has been impossible as yet to reach a definite 
conclusion on this point. 
The greater part of the plain through which the moun- 
tain rises, and which is underlaid by Ordovician strata, is 
mantled by drift which also covers the slopes of the moun- 
tain. 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 thay constitute 
a large part of the whole mass. Perhaps the most remark- 
able of these breccias is that which occurs on St. Helen’s 
island in the harbour 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 sedi- 
mentary 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. The profile accom- 
panying this guide book (p. 36) shows the geological section 
which the tunnel is penetrating. As may be seen from an ex- 
amination of the geological map upon which the position of 
