385 



been lately read to the Society. There is perhaps no class of geo- 

 logical phsenomena in Great Britain which has hitherto remained in 

 more obscurity than that relating to the distribution and origin of 

 superficial gravel, sand, and mud, especially that which has been 

 called diluvium. Mr. Murchison, in his examination of the older 

 rocks of part of Wales and England, has made a great step towards 

 reducing these phaenomena to order, and has thrown so much light 

 upon them that his treatise may be considered not only as one of 

 much local interest, but as likely to contribute powerfully towards 

 the establishment of a general theory of these deposits. He has 

 distinguished between the local drift, or the gravel and alluvium of 

 South Wales and Siluria, and that which he terms the northern drift 

 of Lancashire, Cheshire, North Salop, and parts of Worcester and 

 Gloucester. The surface of the Welsh and Silurian territories is 

 exempt from the debris of far- transported rocks, the alluvium there 

 being derived from the adjacent mountains, while Herefordshire 

 is chiefly covered with debris of the old red sandstone. The au- 

 thor, after giving a detailed description of the drainage of the 

 Teme, Onny, Lug, and Wye, shows that in the valleys of these 

 rivers the loose materials change with each successive range which 

 they traverse, the fragments becoming smaller in proportion as they 

 have been carried to greater distances towards the valley of the Se- 

 vern. It is also demonstrated that there is an evident connexion 

 between the distribution of this ancient gravel or drift and the strike 

 and dip of the strata in the Welsh and Silurian mountains ; and hence 

 it is inferred that the scattering of certain fragments took place 

 during the original upheaving of the mountains. But there are 

 other wide-spread accumulations of sand and gravel in the valleys 

 of the same region, which have partly been due to the existing rivers, 

 and partly to lakes which were drained long after the first emersion 

 of the country from the sea. 



The above-mentioned alluvia differ entirely from another kind 

 of detritus, which is spread over parts of Lancashire, Cheshire, and 

 North Shropshire, and which consists of granites, porphyries, and 

 other hard rocks, similar to those of Cumberland and some of the 

 Scotch mountains. To these, with their associated clay and sand, 

 the author gives the name of the northern drift. It has two di- 

 stinguishing features : first, the occasional occurrence in and upon 

 it of large blocks or boulders of northern origin, sometimes of great 

 size, like the erratics of the Baltic, and none of which ever enter 

 into the region of the Welsh drift ; secondly, the association with 

 it of marine shells of existing species. This last fact was formerly 

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

 at heights of 350 feet above the sea. Sir Philip Egerton has since 

 observed the same shells in sand and gravel, north of Tarporley, in 

 Cheshire, at the height of 70 feet, where they occur at the western 

 base of the Forest Hills, about nine miles from the nearest point of 

 the estuary of the Mersey, But what is still more remarkable, Mr. 

 Trimmer found similar recent marine shells on Moel Tryfane, near 

 the Menai Straits, at the height of 1392 feet above the level of 



VOL. n. 2 B 



