$40 Geological Society. 
been lately read to the Society. There is perhaps no class of geo- 
logical phenomena 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 phenomena 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 ofthe 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 1592 feet above the level of 
