8 
Bulletin of the Natural History Society. 
only the faintest traces can be found in the clay beds, immediately 
spread and occupied the waters of the lake. It is an interesting 
subject of inquiry as to what caused this sudden and important 
change in the condition of the lake and of the surrounding slopes. 
The period during which the lacustrine clays were deposited was 
one of rapid and momentous change in the fauna and flora of the 
lake, being marked by the arrival and increase, and on the other 
hand by the curtailment in numbers and probably the extinction of 
species in quick succession. The part of the deposit above the peat, 
in the accumulation of which a much longer period of time was 
taken than was required for the clays, marks on the contrary 
a quiescent period and was characterized by a slow and gradual 
change in the fauna and flora. 
One cause which may have had an important influence upon the 
deposits made during the period of the Lacustrine clay, was the 
condition of the surface of the land when Torryburn Yalley emerged 
from the sea. The formation of terraces at various levels along the 
sea-coast in this region during the Saxicava period shews that the 
rise of the land from beneath the ocean at this time was not slow 
and gradual, but was effected by spasmodic movements which at 
once, or in a very short space of time, carried large areas of surface 
above the sea. Such a movement between the time of the forma- 
tion of the 100 and of the 50 feet terrace would have exposed the 
steep hill-sides around Lawlor’s Lake, at that time covered with 
marine clay, to the action of atmospheric agents ; there was no mat 
of vegetation, nor covering of trees to absorb and retain moisture, 
and the effect of rains and spring floods in sweeping a turbid deposit 
into the lake can be readily understood. But the sudden cessation 
of this mechanical deposit at No. 5 and its subsequent entire 
exclusion from the lake is not so easily explained. Possibly an 
improvement in the climate may have been influential in promoting 
a more rapid growth of vegetation and in moderating the spring 
floods — possibly artificial dams made on the stream which enters 
the lake may have arrested the muddy waters in their course, and 
prevented them from entering the lake until deprived of the clay 
which they carried in suspension. 
It would appear that the growth of the fresh-water plants was 
subject to much vicissitude during the time when clayey sediment was 
