lis The American Geologist. February, 1894 
the great plain of Lower Canada, varies in thickness from nothing to 
fifty feet or more, and is in many places abundantly charged with Leda 
glacialis in tine preservation, with the valves united and covered with 
epidermis. It also contains locally, as near Ottawa, nodules of its own 
material enclosing numerous fossils, such as leaves and other vegetal rel- 
ics, and especially skeletons of the capelin (Mallotus villosus) and stickle- 
back [Gasterosteus aculeatus), fishes still living in the waters of the lower 
St. Lawrence. Poraminifera and ostracoda also occur in it, and in hold- 
ing these fossils, as well as in its color and texture, it closely resembles 
the blue mud now forming in the deeper parts of the gulf of St. Law- 
rence. Its material has been mainly derived from the wash of the 
Lower Silurian shales of the Quebec and Utica formations, w T hich oc- 
cupy a widespacein the basin. The drift has been carried southwest- 
ward, and grows thinner and finer in that direction. Its western limit 
appears to be where the ridge of the Thousand Isles crosses the St. 
Lawrence river, and where the same ancient rocks cross the Ottawa. 
Deposits referable to the shores of the Leda clay sea, and to the 
estuaries opening into it, are not uncommon, and contain such fresh- 
water genera as Valvata, Paludina, Planorbis, Unhand Ci/elas, with 
occasionally Tellina, the last indicating, at least at times, a brackish 
condition of the water. The beds and fossils of the Leda clay rise to 
an elevation of 425 feet in places, which would carry a deep sea to the 
head of lake Ontario, but "no marine fossils appear to have been found 
on the banks of that lake." 
The Saxicava sand frequently rests upon the upper and often much 
eroded surface of the Leda clay, showing in some places an abrupt and 
in others a gentle transition, in which latter case its lower layers are 
richly fossiliferous. It consists of yellow or brownish quartz sand and 
large travelled boulders, but it rarely contains glaciated stones. It is a 
shallow water deposit, but "must, when at high levels, have been formed 
on the margins of deep seas." 
Into these three deposits have been cut, during the emergence of the 
land, a series of terraces, which at Montreal lie lfcO, 220, 386, 440 and 470 
feet above tide. On Mount Royal a distinct beach lies at 615 feet, and 
travelled Laurentian boulders occur at the very top, at a hight of 700 
feet. A conspicuous line on the lower St. Lawrence between 500 and 
600 feet seems to mark the margin of the sea in which was deposited 
most of the Leda clay, and in this deposit marine shells have been ob- 
served. 
After thus stating the data on which his work rests, the author passes 
on to discuss the climatic conditions of the time. Assuming, as by 
common consent, a high late Pliocene position of the land and a deep 
depression in the Pleistocene, during which the till was deposited, he 
assigns the Leda clay to a late part of the same depression, a third por- 
tion of which is marked by the second boulder-clay, whose boulders 
were carried by floating ice to the summits of the higher hills and 
mountains in eastern Canada and New England, or to a hight of 4,200 
feet. 
