TRANSACTIONS OF THE SECTIONS. 59 



who ascertained that it was nearly pure bismuth, he believed native. Some further 

 specimens satisfied Mr. Tweedy of its being natural, and not artificial, and of its 

 great value. It was found in a mine near Truro, in a large unproductive sparry 

 lode, and only in one spot. 



GEOLOGY AND PHYSICAL GEOGRAPHY. 



On the Upper Silurian Hocks of Denbighshire. Sy J. E. Bowman, F,L.S., 



F.G.S. 



The author commenced by referring to a paper 'On the Upper Silurian Rocks of the 

 Vale of Llangollen,' read by him at the Glasgow Meeting (See Report for 1840, p. 100), 

 and by stating that a partial re-examination of this district, and a survey of that large 

 portion of Denbighshire which lies between the vale of Clwyd and the river Conway, 

 during the present summer, have afforded him additional proofs of the soundness of the 

 arrangement he then proposed, and shown that it is applicable to the whole of Denbigh- 

 shire, notwithstanding some further new appearances still more difficult to reconcile with 

 the typical series of Mr. Murchison. The lofty hills on both sides the great valley of 

 the Dee as high up as Corwen, and all the county of Denbigh west of the vale of Clwyd, 

 except the hills of carboniferous limestone which accompany the upper beds, consist 

 altogether of upper Silurian rocks. On account of the absence of the Aymestry and 

 Wenlock limestones, the changes of sti-uctm-e, and the alterations produced by clea- 

 vage, Mr. Murchison 's subdivisions could not be adhered to. The author therefore 

 proposes to arrange the Denbighshire series in the following descending order :-rT- 



Upper Division. — 1, 2, 3. Green and red sandstones and marly conglomerates, feet. 



with fossils of the Ludlow rocks 100 



4, 5. Blue argillaceous schists, variously affected by cleavage, rarely containing 



fossils of the Ludlow rocks 1000 



Lower Division. — 1. Thin beds of hardened schist without cleavage or fossils... 600 



2. Parallel beds of blue or grey argillaceous shales alternating with others of 

 lighter colour, giving a streaked or banded character to the section. Unfos- 

 siliferous ; horizontal 1500 



3. Coarse slates and flags with large Orthocera, &c. Cleavage nearly coincident 

 with the bedding; often wanting. Lowest beds green, lying u'pon the lower 

 Silurian rocks 1 600 



Total thickness of the Denbighshire upper Silurians 4800 



Upper Division. — 1, 2, 3. These beds are interesting, as containing, under the garb 

 of the old red sandstone, the foSsils of the upper Ludlow rock ; and the more so, since 

 the old red sandstone itself is entirely wanting in North Wales, its nearest point to 

 the beds in question being the outliers of Clun Forest, at the distance of upwards of 

 60 miles. 



4 and 5. These beds are the true equivalents of the upper Ludlow rocks, and 

 generally do not differ much in appearance from the Shropshire type. Though 

 for the most part very barren of fossils, Terebraf.ula navicula, &c. have been found in 

 a few localities. They are often affected by strong cleavage, resembling some lower 

 Silurians ; and however long exposed, usually retain their hardness and colour. A 

 striking proof of their durability occurs in the bed of the Dee under Llangollen Bridge, 

 whose piers rest upon the native rock in the stream. This bridge was built in 1346, 

 yet after the lapse of five centuries the bed of the river is not now more than a foot 

 below what was then its ordinary level. Beds of this age form a broad belt to the 

 south and west of the carboniferous limestone range, and cap many hills of the lower 

 division, north and west of the river Elwy. 



Lower Division. — 1. These thin and apparently siliceous schists occupy the beds of 

 the Dee and the surrounding hills above Llangollen, and insensiblj- graduate down- 

 wards into the next, or middle portion. 



2. These parallel argillaceous shales are of great thickness and extent, and may 

 always be recognised by their streaked or banded appearance, nearly horizontal position, 

 rare occurrence of cleavage, and total absence of fossils. Where the beds separate 

 freely they form good building-stones. This section is extensively developed in the 



