426 NATURAL SCIENCE. dec, 



shells may have been detected in them, can be identified as of the 

 same nature and origin. 



Interbedded with these sands and gravels we find beds of clay 

 and of dirty sand and gravel, but the typical sands and gravels are very 

 clean and require no washing when mechanically separating their 

 mineral constituents by means of sieves of small mesh. 



It must not be thought that shelly-sands and gravels are confined 

 to the high-level localities mentioned. On the contrary, they are to 

 be found at all levels from the sea to the highest mentioned. They 

 vary according to locality and the nature of the surrounding rocks, 

 but they are essentially the same deposits, whether found at fourteen 

 or fourteen hundred feet above the sea. For instance, there is a 

 striking similarity between the shelly-sands and gravels of Howth, 

 near Dublin, which rise from sea-level to about 350 feet above it, and 

 the high-level deposits already described at Gloppa, near Oswestry, 

 1,200 feet above the sea. The foreign rocks they contain differ; at 

 Gloppa they are mostly from the Lake District, Scotland, and Wales, 

 possibly some of the flints may be from Antrim. At Howth the 

 rocks are from the northern parts of Ireland and from the west. 

 They have not, however, been examined to anything like the same 

 extent as in England. 



The low-level glacial deposits are differentiated from the high- 

 level by the greater prevalence of Boulder Clays in the former, and 

 these clays are as a rule distinguished by the still greater prevalence 

 of foreign rocks and their more frequent and intense glaciation. These 

 clays also contain shell fragments, and on washing yield just the same 

 mineral grains and rounded and polished quartz grains. The Boulder 

 Clays cover a very large area of the North-West of England. They 

 are the purest at a distance from mountain masses ; near them, local 

 materials preponderate. 



I have now sketched out the salient features and differences of 

 the high-level and low-level drifts. It would be easy to multiply 

 details, but my object and meaning might then get deeply buried 

 under a mass of what might be called sedimentary information. For 

 anyone who wishes to peruse the subject, there exist papers sufficient 

 to supply continuous reading of the most solid kind for two or three 

 years to come. It would be interesting to know in what state of 

 mind a reader would emerge from such a task, probably his last state 

 would be worse than his first. 



By the earlier and perhaps simpler-minded geologists who paid 

 attention to the subject, such as Trimmer, Ramsay, and Lyell, the 

 position of these shells in the sands and gravels not being explicable 

 by hypothesis of cartage or kitchen-middens, carriage by birds or 

 pilgrims, nor even by waves of translation, were accounted for by 

 submergence of the land by subsidence. They were looked upon, in 

 fact, as marine beds in situ, their heterogeneous character being due 

 •to floating ice, which brought contributions from many localities. 



