lines loaded with boulders, or "ozars." Most of them convey to my mind the impression of ice-action along a slowly subsiding coast, forming successive deposits of stones in the shallow water, and burying them in clay and smaller stones as the depth increased. These deposits were again modified during emergence, when the old ridges were sometimes bared by denudation, and new ones heaped up."

"I conclude these remarks with a mere reference to the alleged prevalence of lake-basins and fiords in high northern latitudes, ast connected with glacial action. In reasoning on this, it seems to be overlooked that the prevalence of disturbed and metamorphic rocks over wide areas in the north is one element in the matter. Again, cold Arctic currents are the cutters of basins, not the warm surface-currents. Further, the fiords on coasts, like the deep lateral valleys of mountains, are evidences of the action of the waves rather than of that of ice. I am sure that this is the case with the numerous indentations of the coast of Nova Scotia, which are cut into the softer and more shattered bands of rock, and show, in raised beaches and gravel ridges like those of the present coast, the levels of the sea at the time of their formation."

2. The Leda Clay.

This deposit constitutes the subsoil over a large portion of the great plain of Lower Canada, varying in thickness from a few feet to 50 or perhaps even 100 feet in thickness, and usually resting on the Boulder clay, into which it sometimes appears to graduate, the material of the Leda clay being of the same nature with the finer portion of the paste of the Boulder clay. Its name is derived from the presence in it of shells of Leda truncata, often to the exclusion of other fossils, and usually in a perfect state with both valves united.

The Leda clay in its recent state is usually gray in colour, unctuous, and slightly calcareous. Some beds, however, are of a reddish hue; and in thick sections recently cut, it can be seen to present layers of different shades and occasional thin sandy bands, as well as layers studded with small stones. It sometimes holds hard calcareous concretions, which, as at Green's creek on the Ottawa, are occasionally richly fossiliferous, but more usually are destitute of fossil remains. When dried, the Leda clay becomes of stony hardness, and when burned it assumes a brick-red colour. When dried and levigated it nearly always affords some foraminifera and shells of ostracoids; and in this as well as in its colour

and texture, it closely resembles the blue mud now in process of deposition in the deeper parts of the Gulf of St. Lawrence.

The lamination of the Leda clay and its included sand layers, show that it was deposited at intervals, between which intervened spaces when currents carried small quantities of sand over the surface. In these intervals shells as well as sand were washed over the bottom, while ordinarily Leda, Nucula and Astarte burrowed in the clay itself. The layers and patches of stones I attribute to deposit from floating ice, and to the same cause must be attributed the large Laurentian boulders, occasionally though rarely seen imbedded in the clay.

The material of the Leda clay has been derived mainly from the waste of the lower Silurian shales of the Quebec and Utica groups, which occupy a great space in the basin of the Gulf and River St. Lawrence. The driftage of this material has been to the South-west, and in that direction it becomes thinner and finer in texture. The supply of this mud, under the action of the waves, of streams, of the arctic currents and tidal currents, and floating ice, must have been constant, as it now is in the Gulf and River St. Lawrence. It would be increased by the melting of the snows in spring and by any oscillations of level, and it is probably in these ways that we should account for the alternations of layers in the deposit.

.*

The modern deposit in the Gulf of St. Lawrence, the chemical characters and coloration of which I explained many years ago,* shows us that the Leda clay when in suspension was probably reddish or brown mud tinted with peroxide of iron, like that which we now see in the lower St. Lawrence; but like the modern mud, so soon as deposited in the bottom, the ferruginous colouring matter would in ordinary circumstances be deoxidised by organic substances, and reduced to the condition of sulphide or carbonate of the protoxide. This colour, owing to its imperme ability, it still retains when elevated out of the sea; but when heated in presence of air, or exposed for some time at the surface, it becomes red or brown. The occasional layers of reddish Leda clay indicate places or times when the supply of organic matter was insufficient to deoxidise the iron present in the mass.

The greater part of the Leda clay was probably deposited in water of from twenty to one hundred fathoms in depth, corres

• Journal of Geological Society of London, vol. v. pp. 25 to 30.

ponding to the ordinary depths of the present Gulf of St. Law rence; and as we shall find, this view is confirmed by the prevalent fossils contained in it, more especially the Foraminifera. The most abundant of these in the Leda clay is Polystomella striatopunctata 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 contemporaneous 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 certain 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 required 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

levels nearer the sea, just as a similar deposit still continues in the Gulf of St. Lawrence. On the whole, then, while we regard this as one bed stratigraphically, we may be prepared to find that in the lower levels the upper layers of it may be somewhat more modern than those portions of the deposit occurring on higher ground and farther from the sea.

Where the Leda clay rests on marine Boulder-clay, the change of the deposits implies a diminution of ice-transport relatively to deposition of fine sediment from water; and with this more favourable circumstances for marine animals. This may have arisen from geographical changes diminishing the supply of ice from local glaciers, or obstructing the access of heavy icebergs from the Arctic regions. At the present time, for example, the action of the heaviest bergs is limited to the outer coasts of Labrador and Newfoundland, and a deposit resembling the Leda clay is forming in the Gulf of St. Lawrence; but a subsidence which would determine the Arctic current and the trains of heavy bergs into the Gulf, would bring with it the conditions. for the formation of a Boulder-clay, more especially if there were glaciers on the Laurentide hills to the north. Where the Leda clay rests on Boulder-clay, which may be supposed to be of terrestrial origin, subsidence is of course implied; and it is interesting to observe that the conditions thus required are the reverse of each other. In other words, elevation of land or sea bottom would be required to enable Leda clay to take the place of marine Boulder-clay, but depression of the land would be necessary to enable Leda clay to replace the moraine of a glacier. I cannot say, however, that I know any case in Canada where I can certainly affirm that this last change has occurred; though on the north shore of the St. Lawrence there are cases in which the Leda clay rests directly on striated surfaces which might be attributed to glaciers; just as in the West the Erie clay occupies this position.

3. The Suxicava Sand.

When this deposit rests upon the Leda clay, as is not unfrequently the case, the contact may be of either of two kinds. In some instances the surface of the clay has experienced much denudation, being cut into deep trenches, and the sand rests abruptly upon it. In other cases there is a transition from one deposit to the other, the clay becoming sandy and gradually pass

ing upwards into pure sand. In this last case the lower part of the sand at its junction with the clay is often very rich in fossils, showing that after the deposition of the clay a time of quiescence supervened with favourable conditions for the existence of marine animals, before the sand was deposited. It is usually, indeed, in this position that the greater part of the shells of our Post-pliocene beds occur; the Saxicava sand being generally somewhat barren, or containing only a few shallow-water species, while the Leda clay is usually also somewhat scantily supplied with shells, except toward its upper layers. Hence it is somewhat difficult to refer a large part of the shells to either deposit, I have however usually regarded the richly fossiliferous deposit as belonging to the Leda clay; and where, as sometimes happens, the clay itself is absent and merely a thin layer rich in fossils separates the Saxicava sand from the Boulder-clay, I have regarded this layer as the representative of the Leda clay.

The Saxicava sand, in typical localities, consists of yellow or brownish quartzose sand, derived probably from the waste of the Potsdam sandstone and Laurentian gneiss, and stratified. It often contains layers of gravel, and sometimes is represented altogether by coarse gravels. It is somewhat irregular in its distribution, forming banks and mounds, partly no doubt in consequence of original irregularities of deposit, and partly from subsequent denudation. In some outlying localities it is liable to be confounded with the modern river sands and gravels. Large travelled boulders often occur in it; but it rarely contains glaciated stones, the stones and pebbles seen in it being usually well rounded.

From the nature of the Saxicava sand, it is obvious that it must be a shallow water deposit, belonging to the period of emergence of the land; and it must have been originally a marginal and bank deposit, depending much for its distribution on the movement of tides and currents. In some instances, as at Cote des Neiges, near Montreal, and on the Terraces on the Lower St. Lawrence, it is obviously merely a shore sand and gravel, like that of the modern beach. Ridges of Saxicava sand and gravel have often been mistaken for moraines of glaciers; but they can generally be distinguished by their stratified character and the occasional presence of animal remains, as well as by the waterworn rather than glaciated appearance of their stones, and pebbles.