Present Aspects of Glacial Geology . 543 



used for brickmaking, there occm-s a great variety of boulders and 

 pebbles, of which those from the English Lake District pre- 

 dominate, consisting of Silurian grits and ashy rocks and lavas 

 from the Volcanic Series, which, if not individually identifiable, are 

 certainly so in the mass. The rocks individually identifiable are, 

 firstly, Eskdale Granite and, secondly, Buttermere and Ennerdale 

 Granophyre. Mixed with these are Carboniferous rocks and, in a 

 lesser degree, Triassic sandstones and shales, the Keuper Marls being 

 identifiable by the casts of crystals of chloride of sodium seen on 

 their surface. Granites from the South of Scotland, and also 

 Silurian greywackes, may be distinguished. 1 These i-ooks, with the 

 exception of the granites, are almost invariably striated, and often 

 intensely so, especially the Silurian grits. The Carboniferous lime- 

 stones are sometimes drilled with holes by boring molluscs, and in 

 one case I found a boulder well striated on one side, and bored at the 

 top and sides with Saxicava with the valves perfect and still 

 remaining in the burrows. From the sand washed out of some of 

 the holes I obtained the fry of Saxicava and of Mactra. 2 



I have shown 3 that though consisting of Boulder-cla}' and sands, 

 the character of the drift differs in various localities, being related 

 to the nature of the rocks in the river-hasins in which it occurs : 

 in one, clay predominates; in another, sand. The sand may lie at the 

 base, in the middle, or on the top, or in all these positions at once. 

 In some places the basal clay is fuller of stones, mostly rounded 

 pebbles, and is of a sandier and harder nature, so as in some few 

 cases, as at Dawpool, on the River Dee, to stand vertically. There 

 are also divisions in the clay in places like unconformity, but 

 probably due to contemporaneous erosion. 



A microscopic examination of the various samples of clay shows 

 that the percentage of sand is commonly much greater than one 

 would expect it to be. 4 The sand-grains, as a rule, are much worn 

 and rounded, and no doubt they have commenced as rounded grains, 

 in many cases being derived from the Trias, but they are often much 

 more polished than any grains I have taken direct from Triassic 

 sandstone, being perfectly smooth and transparent. Also, there are 

 to be seen very minute polished pebbles of hard drift rocks. The 

 shell fragments often, but not alwa}'s, show signs of much attrition. 

 These deposits rest upon a floor of Triassic or, in some cases, 

 Carboniferous or Permian rocks, which have a system of valleys 

 not always corresponding with the present orographic system. 



The Drift has by filling up the valleys sometimes changed the 

 pre-Glacial course of the rivers, so that in the case of the Mersey 

 it now runs in the rock-channel at Runcorn Gap, instead of along 

 the deeper rocky valley buried below the town of Widues. The 

 pre-Glacial scenery was deeper-cut and bolder than the usually soft 



1 Q.J.G.S. 1884. 



2 Nature, vol. xl, p. 216. 



3 Q.J.G.S. 1883, vol. xxxix, pp. 83-132. 



4 See Davies and Reade, Proc. Liverpool Geol. Soc. 1894-5. Reade, "Drift 

 Beds of Moel Tryfaen " : ibid., 1S92-3 ; etc. 



