188 THE ICE AGE IN CANADA. 



with the Saxicava sand, and apparently resting on the 

 terraces cut out of the older clays. This is the arrange- 

 ment which prevails throughout this part of Canada. It 

 is modified by the greater or less relative thickness of the 

 boulder-clay and Leda clay, by the irregular distribution 

 of the overlying sands, and by the projection through it 

 of ridges of the underlying rocks. 



The section at Trois Pistoles may be represented as 

 follows in descending order : 



1. Sand and Gravel, capping the terraces cut in the 



previous deposits, and forming slight ridges or 

 eskers in some of the lower levels. It contains on 

 the lower terraces a few shells of Leda and Tellina. 

 At the bottom of this deposit there are seen in 

 places many large boulders of Laurentian and 

 Lower Silurian rocks, resting on the Leda clay 

 below. 



2. Leda Clay, exposed in the railway cutting and seen 



also in the edge of the second terrace. Thickness 

 one hundred and twenty feet or more. It holds 

 a few large boulders and shells of Leda glacialis 

 the latter uninjured and with the valves united. 



3. Boidder-clay, or hard gray till, with boulders and 



stones. Seen in a mill-sluice near the bridge, and 

 estimated at twenty feet in thickness at this place, 

 though apparently increasing in thickness farther 

 to the westward. 



4. Shales of Siluro-Cambrian age, seen in the bottom of 



the river near the bridge. They are smoothed 

 over, but show no striae, though they have 

 numerous structure lines which might readily be 

 mistaken for ice-striae. 



