166 Stewart— Mollusca of the Boulder Clay. 



The Molluscan shells occurring in the Boulder Clay are not numerous ; in 

 most cases they are only found by patient searching, and then only in 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. The fauna of the 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 in 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 

 bear 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 the 

 dropping down of large boulders, and the result that remains is the unstratified 

 till, with its occasional entire shells and more numerous 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 



