D. MACKINTOSH ON HIGH-LEVEL MARINE DRIFTS. 



365 



shelly gravel and sand, as already stated, there would appear to be a 

 zone consisting of loam, with angular or subangnlar stones and no 

 shells ; but this zone is not uniformly continuous, as on Halkin 

 Mountain (Sec. III.), and possibly in other places ; it is varied by 

 accumulations of more or less rounded gravel and sand. Below the 

 third zone (order descending), and extending down to the existing 

 sea-level, there is a zone of horizontally alternating clay, and 

 angular and rounded gravel and sand, with boulders and shells*. 



Causes of the horizontal Discontinuity of the Middle Zone of 

 rounded Gravel and Sand. — The question may naturally be asked, 

 How is it that the rounded and shelly drift of Moel Tryfan, 

 Prondeg, and Macclesfield Forest are horizontally separated by 

 areas in which the drift is angular ? We may, I think, be enabled 

 to arrive at some explanation of this fact by supposing that rounded 

 drift on this horizon may exist in many places under a covering of 

 Boulder-clay, which may have been wholly or partly left during 

 the rising of the land ; but there are other considerations which 

 might account for the local absence of rounded drift, one of them 

 being that the sea, by merely lingering for a time near the same 

 level, might not be able to accumulate rounded drift without being 

 assisted by other favourable circumstances or conditions (positive 

 or negative) such as the following : — (1) a sea- coast exposed to 

 wave-producing winds ; (2) the previous or contemporaneous 

 accumulation of heaps of angular debris by melting or stranding 

 icebergs or coast-ice, frost, rain, gravitation, &c. ; (3) the absence 

 of steep slopes which would so facilitate the washing-down of pre- 

 existing angular debris below the reach of wave- action as to pre- 

 vent the accumulation of rounded stones f ; (4) the absence on sea- 

 beaches of clay, which would be unfavourable to the rolling of 

 stones ; (5) the prevalence of stones susceptible of being easily 

 rounded; (6) the existence of rapid currents capable of rolling 

 stones below the limit of wave-action ; (7) the preservation of 

 relative levels of the differently characterized zones during the 

 emergence of the land ; (8) the n on -alteration of the character of 

 the zones by deposits left during emergence ; (9) the non-removal 

 of preexisting rounded gravel and sand from valleys and slopes by 

 glaciers during or after emergence. The last consideration is very 

 important ; for, according to Professor Bamsay (to the great accu- 

 racy of whose observations in North Wales I can bear humble 

 testimony), the marine drift of the northern valleys was ploughed 

 out by the second glaciation of the country. 



* It might unnecessarily complicate the main subject of this paper were I to 

 refer particularly to a zone of fine rounded gravel and sand, with no boulders, 

 at a low level in the plain of Cheshire and part of Lancashire (which I believe 

 was deposited during the last stage in the rising of the land), or to the upper 

 Boulder-clay at comparatively low levels, which I believe was the result of 

 a re-submergence of the land. I lately found proofs of a land-surface between 

 the upper Boulder-clay and underlying boulderless sand in excavations under 

 Crewe railway-station, an account of which will soon be published. 



t On the very steep slope of Mynydd Mawr, which is nearly opposite to the 

 marine drift on Moel Tryfan, the stones are all angular. 



