RAMSAY DEIFT-PEEIOD OF CANADA. 201 



On the south side of the river the country is low, being formed of 

 Silurian strata chiefly covered with drift from the Laurentine chain ; 

 and the vast quantity of boulders and smaller stones that cover the 

 land help to impress on it a poor agricultural character. 



Approaching Montreal, the gneissic mountains recede to the north- 

 west ; and both banks of the river are low, except where an 

 occasional boss of greenstone pierces the Silurian strata. Montreal 

 Mountain, about a mile behind the city, is one of these, rising 

 boldly out of the terraced drift of the plain. 



This drift consists of clay, with Laurentian boulders and boulders 

 of greenstone from the mountain, both mixed with subangular 

 gravels of Utica slate and Trenton limestone, which formations rise 

 on its flanks. Many of the boulders and smaller stones are grooved, 

 or more finely scratched, in a manner undistinguishable from the 

 scratched stones of the British and Alpine drift or of Alpine glaciers. 

 We are indebted to Dr. Dawson of Montreal for three important 

 subdivisions of the superficial deposits, — ^namely, 1st, at the base, 

 lower boulder- clay and gravel ; 2ndly, an unctuous clay, with many 

 marine shells, called by him the *' Leda-clay" {Leda Portlandica), 

 on which lie, 3rdly, beds of gravel and sand, with shells, one of the 

 most common of which is Saxicava rugosa. These subformations 

 occasionally pass into each other where they join. The Saxicava- 

 sand he considers to have been a shallow and sublittoral deposit ; the 

 Leda-clay to have been accumulated at depths of from 100 to 300 

 feet or more ; and the true boulder- clay to have been formed at an 

 earlier period of subsidence, during which an ocean spread over the 

 greater part of IS'orth America. I shall have occasion to show that 

 at one tune this sea was, in places, probably over 3000 feet in depth. 

 The section (fig. 1)* across the drift, which I drew at Montreal, 

 nearly agrees with Dr. Dawson's, with the exception that I show five 

 terraces in the drift, while he gives two. Their number may vary 

 in different localities ; but they have certainly been formed during 

 the last emergence of the country, each terrace indicating a pause in 

 elevation ; and in a great degree the shells of the upper strata lie 

 in a debris of remodelled drift. The two upper terraces, to the left 

 of Dorchester Street, correspond to Dr. Dawson's Leda-clay and 

 Saxicava-sand. 



Between the lowest terrace and the river there is a broad marsh, 

 including patches of recent freshwater shells. It is part of the old 

 course of the St. Lawrence ; and on its surface (the lighter drift 

 having been removed) the boulders that once studded the clay 

 have been concentrated. Similar terraces occur on the banks of the 

 Ottawa. The country is strewn with boulders of gneiss and meta- 

 morphic limestone, from the neighbouring Laurentine chain, mixed 

 with more local debris ; and here also it seemed, in several cases, as if, 

 by removal of the lighter material, the boulders were more concen- 



* For the Silurian geology of this diagram, I am indebted to the descriptions 

 of Sir Wm. Logan. 



