630 REV. THOMAS BROWN ON THE ARCTIC SHELL-CLAY 



only have to consider at what point of depth all the above species could meet. 

 Some of them have a wide range, like the Crenella laevigata, for example, which 

 is found at all parts of the sea-bottom, from 5 to 200 fathoms. Others, like the 

 Natica grcenlandica, have only been got from deep water. On the whole, how- 

 ever, our information as to the range of shells will hardly allow us to do more 

 than form an approximate opinion. The species on which the conclusion must 

 mainly turn is the Leda truncata, which is beyond all comparison the shell of 

 the deposit. At Spitzbergen, Sec, it is described as ranging from 5 to 30 fathoms; 

 and between these two points, therefore, the degree of depth must lie. But, look- 

 ing to the shells with which it is associated, it must evidently lie somewhere about 

 the lower portion of its range. The Thracia myopsis, for example, is found at 60 

 fathoms, and ranges from that down to 200. The new species of Yoldia was 

 dredged only in 100 fathoms. The presence of such shells would seem to indi- 

 cate that we are to fix the point somewhere towards the lower range of the Leda, 

 say 160 feet. This is the deposit which is now lying more than 40 feet above 

 the sea ; and we can hardly be wrong in concluding, that at the time these shells 

 lived the level of the land was at least 150 to 200 feet lower than now. 



V . — Bo ulder- Clay. 



The lowest stage of these deposits is that of the boulder-clay, which, both at 

 Errol and Elie, is found lying beneath the Arctic shell-clay, and resting imme- 

 diately on the rock. It would seem that this lowest deposit, so long an enigma, 

 has at last yielded up its secret, — that it is a land deposit, formed at the period 

 when Scotland, like Spitzbergen, lay beneath an immense covering of ice, which 

 wrapped the whole face of the country, hill and dale. Underneath such a cover- 

 ing, possibly thousands of feet in thickness, the rocks would be ground down, 

 and the boulder-clay formed. And thus the absence of fossils is accounted for, 

 inasmuch as none of our usual forms of life could exist beneath such an ice-sheet. 

 And thus we see also how the clay is so peculiarly hard and untractable. This 

 hardness is very strikingly noticed wherever the true boulder-clay is dug into, 

 as at the mantrap, near Leith, or in the neighbourhood of Greenock, where, in the 

 excavations at the harbour, I found them blasting it with gunpowder. It would 

 seem as if the pressure of immense ice-masses passing over it had compacted it 

 into such hardness. This feature was very marked in the railway cutting at Elie. 



As to the relations of the boulder-clay and the Arctic shell-clay, there are two 

 questions to be considered : — 



1. Is the boulder-clay an antecedent formation, and did the Arctic shell-clay 

 come to be formed only after the formation of the boulder-clay had ceased ? 



Both at Elie and Errol the shell-clay rests on the other ; and it is, of course, 

 plain, that the particular portion of the boulder- clay there present is older than 

 that portion of the shell-clay. But we must take care how we extend this 



