148 G. F. MATTHEW ON A METHOD OF 
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 Characeæ 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 alg@; 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 
Naiadacee 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 Characeæ 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 abun- 
dance 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 : 
— 
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