333 



Northern Drift. — Detritus differing entirely from that which covers 

 Wales and Siluria, is spread over large parts of Lancashire, Cheshire, 

 and N. Shropshire, ranging up to the edges of the region above 

 mentioned. The materials of this drift consist of granites, porphyries, 

 and other hard rocks, which have been derived from the mountains of 

 Cumberland, a few perhaps from those of Scotland. The drift further 

 contains much sand and clay, with many pebbles of smaller size, 

 which varies exceedingly in different districts. Thus, in N. Salop, 

 near the great outlier of lias, described by the author*, fragments of 

 that formation are added to the mass, and as it advances to the south 

 the materials become still more varied ; the fragments, however, of 

 the northern granite and porphyries always existing to identify the 

 drift. Its distinguishing feature is the reoccurrence at intervals of 

 large blocks or boulders, of northern origin, a large proportion of 

 which lie at various heights on the slopes of the mountains skirting 

 the N. Welsh coal-field, and encumbering the northern flanks of the 

 Wrekin and of Haughmond Hill ; while a few have been propelled to 

 the edge of the Silurian rocks south of Shrewsbury. They prevail in 

 vast quantities in the high inland district between Wolverhampton and 

 Bridgnorth, from which latitude they begin to diminish in size ; but 

 coarse gravel, composed of the same materials, is prolonged south- 

 wards like the tail of a delta through Worcestershire, until it dies 

 away in the fine silt and gravel of the Vale of Gloucester. Not a 

 fragment of any such detritus, enters into the region of M^elsh and Si- 

 lurian drft; but in the environs of Shrewsbury certain mounds of the 

 latter are capped by clay and boulders of the northern drift, which 

 is thereby shown to be of subsequent formation. The best proof of the 

 recency of the epoch during which this northern drift was accumulated 

 is, that it contains sea shells of existing species. These were formerly 

 noticed at Preston, in Lancashire, by Mr. Gilbertson ; and by the 

 author at the height of 350 feet above the sea. In Cheshire they have 

 been observed by Sir P. Egerton at heights of about 70 feetf. Mr. 

 Trimmer has cited similar shells on Moel TryfaneJ, now ascertained to 

 be 1392 feet above the sea, and has recently detected them near 

 Shrewsbury. Mr. Murchison has collected evidence of their diffusion 

 over a wide area in Shropshire, tracing them at intervals from Maring- 

 ton Green, N.W. of Shrewsbury, by the Wrekin and Wellington, to 

 the high grounds between Bridgnorth and Wolverhampton, at least 

 ,60 miles inland, and at heights varying from 300 to 600 feet. He has 

 also been enabled to add several species to those mentioned in any 

 former list. These shells having been examined by good conchologists 

 (including Dr. Beck, of Copenhagen,) prove to be identical with spe- 

 cies now inhabiting adjacent seas, viz. Buccinurn reticulatum, B. un- 

 datum, Deniulium entalis (Linn.), Littorina littorea, Tellina soldula? 



Venus , Astarte , Cardium tuberculatum, C. edule, Cyprina 



islayidica, Turritella ungulina (Beck), [Turbo ungulinus, Linn.,) Donax 

 or Mactra. 



It was a prevalent the belief that large boulders were usually lodged 



* Proceedings of the Geoloeical Society, Vol, If. p. 114. 

 i'lbid., Vol. II. p. ISii '^ % Ibid., Vol. I. p. 3,3i. 



VOL, U X 



