36 THE CANADIAN NATURALIST. [Vol. Vl. 



ponding to the ordinary depths of the present Gulf of St. Law 

 rence ; and as we shall find, this view is confirmed by the preva- 

 lent fossils contained in it, more especially the Foraminifera. The 

 most abundant of these in the Leda clay is Polystomella stria to- 

 jnmctata var. arctica, which is now most abundant at about 

 twenty-five to thirty fathoms. Since, however, the shallow-water 

 marine Post-pliocene beds extend upwards in some places to a 

 height of six hundred feet on the hills on the north side of the 

 St. Lawrence, it is probable that deposits of Leda clay contem- 

 poraneous with these high-level marine beds were formed in the 

 lower parts of the plain at depths exceeding one hundred 

 fathoms. 



The Western limits of the Leda clay appear to occur where 

 the Laurentian ridge of the Thousand Islands crosses the St. 

 Lawrence, and where the same ancient rocks cross the Ottawa ; 

 and in general the Leda clay may be said to be limited to the 

 lower Silurian plain and not to mount up the Laurentian and 

 metamorphic hills bounding it. Since, however, the level of the 

 water, as indicated by the Terraces in Lower Canada, and by 

 the probable depth at which the Leda clay was deposited, would 

 carry the sea level far beyond the limits above indicated, and 

 even to the base of the Niagara escarpment, we must suppose, 

 either — (1) that the supply of this sediment failed toward the 

 west ; or (2) that it has been removed by denudation or worked 

 over again by the fresh waters so as to lose its marine fossils ; or 

 (3) that the relative levels of the Western and Eastern parts of 

 Canada were different from those at present. As already stated 

 there are indications that the first may be an element in the 

 cause. The second is no doubt true of the clays which lie in the 

 immediate vicinity of the lake basins. There are, as yet no cer- 

 tain evidences of the third ; but the facts previously stated on 

 the authority of Dr. Newberry, lend it some countenance ; and 

 detailed surveys of the Terraces and raised beaches would be 

 required to determine it. I believe, however, that much more 

 rigorous investigations of the clays of Western Canada are re- 

 quired before we can certainly affirm that none of them are 

 marine. 



I believe the Leda clays throughout Canada to constitute in 

 the main one contemporaneous formation. Of course, however, it 

 must be admitted that the deposit at the higher levels may have 

 ceased and been laid dry while it was still going on at lower 



