Laaistrine Formation of Torryburn Valley. 7 
attributed the presence of such species as Balanus crenatus^ B. 
Hameri and Mytilus edidis, common in the Upper Leda-clay but 
which are also found in tlie marine beds immediately beneath the 
lacustrine clay of Lawlor’s Lake. The passage from the marine to 
the fresh-water part of the clays, though not obvious to the eye, can 
be detected by a change in the s})ecific gravity of the clay, and this 
will be found to be a useful method of separating the different parts 
of a continuous deposit, partly of salt-water and partly of fresh- 
water origin. At 16 the clay is about 1-10 lighter* than the 
marine clay beneath, and in the next inch No, 2a is reduced so as 
to be only two thirds of the weight of la. This change of density 
in the clay is due to the introduction of organic matter, chiefly low 
forms of vegetation which must have grown abundantly in the lake 
after the exclusion of the sea. The vegetable matter is mostly 
cellular tissue and not readily noticed except for the changed color 
of the clay ; but its effect upon the weight of the clay can be at once 
seen by comparing the speciflc gravities, of Nos. 3 and 4, which are 
reddish clays like No. 16, with that of the marine bed No. la 
which is only about a flfth heavier than these (Nos. 3 and 4.) The 
large addition of vegetable matter in No. 5, by which the color of 
the clay has been changed to olive grey, has a decided effect on the 
specific gravity ; in the lower part of this division the weight is 
one quarter less than the standard of the marine clays, and in the 
upper part is not more than half. These weights do not express the 
full difference in the actual weight (less the water) of these different 
parts of the deposit when fresh ; for in drying No. 5 lost nearly one 
quarter of its bulk, and No. 2 also lost in volume considerably. 
The shrinkage is no doubt due to the large quantity of cellular algae 
which these deposits contain. Such a marked difference between 
the weight of marine and lacustrine clays in the successive, or 
alternating beds of a deposit, is not to be looked for in larger basins, 
but I have no doubt the rule will be found to hold good to an 
appreciable degree in almost any fresh-water deposit formed in a 
basin of moderate dimensions. 
Div. 5 marks an important epoch in the history of the lake, for 
at this period the influx of clay carried down by streams into the 
lake was arrested, and the molluscan fresh-water fauna., of which 
♦The weights given are those of air-dried samples of the deposit coarsely pulverized. 
