190 Correspondence — Mr. 8. G. Perceval. 



of local Lower Boulder-clay. The more the subject is investigated, 

 the more oue becomes convinced that the Upper Boulder-clay is the 

 newest glacial or interglacial deposit in the N.W. of England. 



The shell-bearing gravel-and-sand around Macclesfield, which ac- 

 cording to Mr. Sainter (Geol. Mag., Vol. II., pp. 366, 368) ranges 

 from 600 to 1200 feet above the sea-level, is I believe an upward 

 extension of the middle drift of the plain of Cheshire. The deposit 

 on Moel-y-Tryfan is not so finely stratified, and the stones are not so 

 much rounded as in the gravel at lower levels ; but it agrees with 

 the gravel-and-sand of Anglesey in the large boulders being found 

 at its base (though a few are scattered through its mass), and with 

 the Cheshire and Lancashire middle drift in the character of many 

 of its erratics, including Eskdale and Criffell granite, chalk-flints, 

 etc. Mr. Darbishire says it is capped with clay (Geol. Mag., Vol. 

 II., p. 295) ; though this I missed seeing. 



The following is the sequence of the drift deposits of the N.W. of 

 England and a part of Wales (order descending) : — 

 Eed clay, with grey or blue partings ; rather few stones and exceedingly few large 

 boulders ; more or less marly in its lower part ; extensively used for bricks. 

 Maximum thickness unknown. 

 Gravel and sand, with subordinate beds of clay and loam. Maximum thickness in 

 Cumberland 120 feet; at Gresford, near Wrexham, 150 feet; at Kersal Moor, 

 Lancashire, 200 feet (Hull).i 

 Madder-brown clay, with subordinate beds of laminated loam, seams, pockets, and 

 lenticulations of sand; numerous stones, and at intervals many large boulders; 

 vertical or oblique fractui'es ; often rudely stratified ; graduating into a still 

 harder, more gritty, and stony clay, with a tendency to arched stratification, in 

 the neighbourhood of the hills (Pinel) ; in general not well adapted for bricks. 

 Maximum thickness at Lindal, Furness, 120 feet. 

 Blue or greyish-blue clay, with many stones and at intervals many boulders. Maxi- 

 mum thickness at Colwyn, N. Wales, not less than 60 feet. 



The above drifts have been found to be shell-bearing with the ex- 

 ception of the blue clay and the pinel. The best sections may be 

 seen on the sea-coast. D. Mackintosh. 



ME. W. S. MITCHELL ON THE "DENUDATION OF THE BATH 

 OOLITE." 



Sib, — I believe Mr. Mitchell is of opinion that the hills of Bath 

 Oolite were simply old coral-reefs, and did not owe their form to 

 denudation. (See Quart. Journ. Geol. Soc, 1871, vol. xxvii., p. 228.) 



I was staying near Bath in the Spring of 1869, and discovered, 

 in a shallow excavation on the left of the road on Kingsdown, 

 beyond Bathford, some beautiful specimens of Oolitic Corals — the 

 finest which have been obtained from the neighbourhood of Bath — 

 the best of which I gave to Mr. Charles Moore, for the Bath Museum. 

 They consisted of several genera and species, and occurred at one 

 portion of the excavation ; the space which they occupied being a 

 few yards across, and appearing to be a section of a small coral 

 reef, bounded on each side by the usual limestone. It is possible 

 this bed increases in width, vertically. 



Corals appear to be of rare general occurrence in the Bath hills ; 



1 In the Vale of York this formation is associated with deposits of finely laminated 

 clay of various colours. 



