350 



D. MACKINTOSH ON HIGH-LEVEL MARINE DRIFTS. 



(3) Shells have not been found in Eskdale on the old sea-coasts from 

 which the granite erratics on Moel Tryfan were transported when 

 the land was deeply submerged (see 11). 



9. Cause of the Absence of similar Deposits on the Lower Slopes of 

 the Hill, — It has often been remarked that beach-like deposits with 

 shells have not been found on Moel Tryfan excepting towards the 

 summit of the hill. It is perhaps too early to speak positively on 

 this point, as there is a bare possibility of patches of such deposits 

 being yet discovered. But the cutting for the railway above Bryn- 

 gwyn, which extends from near the base to near the summit of the 

 hill, shows no trace of laminated sand and rounded fine gravel, but, 

 on the contrary, is everywhere a Boulder-clay, or clayey loam, with 

 stones, chiefly angular or sub angular. The drift on the north side of 

 the hill, so far as revealed by cart-roads, is somewhat similar to that 

 above Bryn-gwyn, excepting towards the summit, where there are 

 patches of gravel and sand interstratified with clay and loam. The 

 cause of the difference in the deposits near the summit and lower 

 down cannot (at least principally) be a difference in the sources of 

 supply of the lower and higher drifts, because the materials for the 

 elaboration of fine sand and gravel exist on the hill-slopes at low as 

 well as at high levels. The most probable explanation seems to be 

 that the submergence of the lower slopes of Moel Tryfan went on too 

 rapidly to allow sufficient time for the accumulation of well-rounded 

 beach shingle and sand (see sequel). 



10. Moel-Tryfan Shells and Erratics not pushed up hill by Land- 

 ice. — It has been asserted (though this is not the general opinion) 

 that all the shells, along with the erratic stones, were pushed out 

 of the bed of the Irish Sea as far south as Moel Tryfan, and then 

 up the hill-slopes nearly to the summit by land-ice. But an inti- 

 mate acquaintance with the character of the Moel-Tryfan deposits 

 precludes this idea ; for, if it were a true explanation, the shells and 

 erratic stones would have diminished in number the higher up they 

 were pushed. But, on the contrary, the shells and erratics in the 

 drifts near the sea are fewer in number than on Moel Tryfan. 

 This theory would likewise require to invest the land-ice with the 

 power of rounding the pebbles derived from the upper part of the 

 hill, and laminating the sand and fine gravel ; for it ought to be 

 remembered that though the sand and gravel are, in places, much 

 contorted on Moel Tryfan, the contortion was evidently, in many 

 instances, produced after their accumulation. 



11. Shells found by the Author. — As my main object was not to 

 look for shells, only the following nine species were named for me 

 by Dr. Gwyn Jeffreys out of a number of fragments I collected :— 



Cardium echinatum. Tellina balthica. 



edule. Mactra solicla, var. elliptica. 



Cjprina islandica. Saxicava rugosa. 

 Astarte sulcata. Purpura lapillus. 

 borealis. 



It is quite consistent with the remark made in section 8, that- 

 some of the Moel-Tryfan shells may have been brought along with 



