VI.-On a Method of Distinguishing Lacustrine from Marine Deposits.

By G. F. MATTHEW.

(Read May 25, 1883.

While examining samples of the recent deposit of the Torryburn Valley, near Saint John, N.B., the writer had occasion to test the specific gravity of the layers from different parts of the formation, and found that there was a constant relation between the weight and the physical conditions under which the several parts of the deposit were formed. The most decided change in the specific gravity was noticeable, where marine gave place to fresh-water clay; and again, where the latter were succeeded by lake peat and freshwater marl.

The lacustrine beds of this formation were deposited in a sheltered basin surrounded by hills, and all the conditions were favourable to the preservation of minute organic matter. The underlying clays which are undoubtedly marine can easily be recognized as such by the presence in them of such shells, as Mya truncata, Mytilus edulis, Macoma Groenlandica, Balanus crenatus, &c. The over-clays have no such shells, though rare fragments, which may have belonged to Mytilus edulis and Mya truncata, are occasionally found in the lower layers near a point where a brooklet enters the lake: these, however, appear to be accidental, and to have been introduced into the lacustrine clay by the washing away of the marine clay along the course of the stream after the elevation of this clay above the sea-level.

In the texture and general appearance of the clay at the border of the present lake there is very little difference between that which is marine and the later clay of freshwater origin; but, on levigating the latter, a considerable quantity of finely divided organic matter was obtained, and was evidently an important constituent. The lessened gravity of this part of the deposit can thus be accounted for, long before the presence of a lacustrine fauna gives the more obvious proof of a change from marine to lacustrine conditions. This, in the case of the Torryburn clays, was not obtained until about one twelfth of the whole fresh-water deposit had been passed through (from the bottom upward); here, however, it was but the slightest indication of molluscan life which might easily have been overlooked, and the molluscan fauna did not appear in force lower down than one-eighth from the bottom.

On investigation of the finely divided organic matter of the lacustrine clay, it was found to be almost entirely composed of cellular tissue and, where this most abounds, the clay possesses a dark olive-gray hue. The original colour of the unchanged argillaceous sediment, as may be seen both in the marine clay and in such parts of the fresh-water clay as are most deficient in organic matter, having been reddish brown. Twigs, leaves, and small branches of trees and shrubs are scattered through the lacustrine clay, but do not seem to have had much influence in altering its general colour, the elimination of the red colour being apparently due to the finely divided organic matter. It seems highly probable

that this finely divided organic matter consists of the remains of Confervoid Algae which would thus appear to have grown vigorously in the ponds which dotted this valley after the recession of the sea.

The Characea helped to increase the lacustrine vegetation, but do not seem to have been so directly responsible for the changed colour of the clay as the algæ; for their abundance is not in direct relation to the olive tint of the lacustrine clay, nor to the decreased specific gravity of this part of the clayey deposit. Leaves and stems of the Naiadacea also played a part in effecting this change of colour, but they influenced only the upper layers.

At the summit of the clay all these causes combined to determine a strong olive colour in the deposit, which here passes into a peaty mass, containing, beside the forms of vegetation named above, remains of a terrestrial flora in which seeds and leaves of grasses and leaves of shrubs and trees abound.

At the top of the pure lake peat, another element affecting the specific gravity is introduced. This is the fresh-water molluscan fauna, which at this time spread throughout the lake and at once changed the constitution of the sediment. The cellular organisms. were to a great extent removed, and Potamogeta abounded for a while. The Characea also were exceedingly plentiful. But although molluscs now abounded in the lake, the specific gravity of the sediment is apparently no greater than that of the peat below, owing, no doubt, to the small cavities in the deposit, caused by the hollow shells of the molluscs. Through the instrumentality of these creatures a large amount of carbonate of lime was deposited from the waters of the lake, and added to the sediments at its bottom.

Though the shells of the fresh-water molluscs serve to increase somewhat the specific gravity of the upper part of the lacustrine deposit, this is still much less than that of the marine clay in which the absence of organic matter, especially of vegetable origin, is a decided characteristic. The red colour of this-the Leda clay-is therefore strongly marked in the south-eastern part of New Brunswick where this deposit was produced chiefly by the abrasion of the red shales of the Lower Carboniferous formation. An exception to this red colour is found in certain dark-colored bands and layers where there is a great abundance of marine organisms; but even these do not posses the low specific gravity which characterizes the fresh-water beds of the Torryburn lakes. The test of comparative weight will be found useful in distinguishing these two classes of clays (which will, no doubt, be found in close geological sequence in sheltered inland basins over large areas of this part of Canada and Maine) where, from the absence of molluscan remains, the question of the origin of the clay might otherwise be doubtful.

The weight of the samples was tested in the usual way, by weighing in air and in water, and the following table gives the average result:

*This layer shrank one-fifth in drying; if it had retained its original size the specific gravity would have

been less. All the samples were air-dried before weighing.