Stewart — Mollusca of the Boulder Clay. 
i 66 
The Molluscan shells occurring in the Boulder Clay are not numerous ; in 
most cases they are only found by patient searching, and then only m a frag- 
mentary condition ; but in a few instances they are less rare, and include speci- 
mens in a perfect state. Astarte sulcata , A. elliptica , A. compressa, and Leda 
pernula are the prevailing and characteristic forms ; the most widely diffused 
and the most numerous in examples. Fragments of these shells are to be found 
almost everywhere, and perfect shells are frequently met with. Leda pygmcea 
is also remarkable. Though a very small and fragile shell, it is usually found 
in a perfect state, with the two valves united. The presence of perfect shells 
of Leda was known long since to General Portlock, and forced him to the same 
conclusion as arrived at by the Author, that the Boulder Clay is a marine 
sedimentary deposit. The absence of stratification has led most geologists to 
the conclusion that the Boulder Clay is an accumulation resulting from land ice, 
and the occurrence in it of so many marine shells seems to have been either 
unknown or overlooked. Thefauna ofthe Boulder Clay is not by any means rich, 
and this is what we might expect from the rigour of the climate of that period. 
Nevertheless it is a marine fauna ; and there is unquestionable evidence m the 
case of several of the species that they truly belong to the deposit, and were not 
drifted into their present position from a distant locality, nor yet are they fossils 
derived from an older bed. 
From what has been stated, it will be seen that we must account for the 
want of stratification in some other way than the land-ice theory, as the presence 
of even one marine shell unquestionably belonging to the deposit renders that 
theory invalid. The supposition of a general ice-cap can derive no support 
from the Boulder Clay. The underlying rocks are very often glaciated, and 
hear strise which tell of ice that was moving over them from the north, and this 
is corroborated by the stones contained in the clay, which also tend to show a 
similar ice movement. We must, therefore, conclude that the clay was rapidly 
deposited from water-borne ice, and that the ice, with its burthen of clay and 
stones, was impelled by currents having a more or less southerly direction. On 
the floor of the sea lived a scanty boreal fauna. Many of the shells were 
broken into fragments by the ice grounding in the shallow waters, or by t e 
dropping down of large boulders, and the result that remains is the unstratified 
till, with its occasional entire shells and more numer,. us fragments 
The Boulder Clay forms the subsoil over the greater part of our district 
adding much to the fertility of the land. It is best seen in river banks, because 
the streams have made sections of the strata, and thereby exposed it to our 
view. It has, consequently, been often considered as only, or mainly, deposited 
in sheltered hollows ; but it is found 'that where artificial cuttings have been 
