Aug. 26, 1875J 



NATURE 



353 



range when at a lower level, its partial removal being conclu- 

 sively shoAvn by the remnants that still cling to the steep face of 

 the northern and southern flanks of the Mendips. 



This Conglomerate is composed entirely of greater or lesser 

 fragments of the older rocks composing the hills, and is the 

 result of the denuding action of the sea that deposited the 

 Keuper beds. This marine denudation took place when the 

 entire area occupied by the Mendips and Coal-basin underwent 

 depression, the Dolomitic Conglomerate and sandstones accumu- 

 lating /re? rata with the depression and consequent destruction of 

 the rocks offered for resistance. This conglomerate, the '* over- 

 lie " of the coal-miners of the Bristol basin, although visible only 

 upon the Palaeozoic rocks surrounding the coal-bearing area, is 

 nevertheless entirely spread over them, and beneath the New 

 Red Sandstone that occupy nearly the entire area from Tort- 

 worth to the soutliern flanks of the Mendips, its presence being 

 marked by the marls and sandstones of the Keuper, the Lias 

 limestones, and in other places the Oolitic rocks that lie within 

 the Coal-basin, especially along its south-east border from Bath 

 to Wells. We have no physical evidence more convincing of 

 denudation, elevation, and depression over large areas of tlie 

 earth's surface than what we can witness so easily and study so 

 advantageously in the Mendip Hills ; for this conglomerate rock 

 here defines the limits between Mcsozoic and Palceozoic times : 

 the highly inclined Old Red Sandstone forms the nucleus of the 

 chain, the Carboniferous rocks resting upon it ; and the Coal 

 Measures in conformable succession to the latter were all in- 

 durated, metamorphosed, elevated, and thrown into folds long 

 prior to the time when, under slow depression, destruction, and 

 denudation, the Dolomitic Conglomerate was laid down by the 

 Triassic sea — the resultant of wave forces along a coast-line 

 which was then the Mendip range, its shingle and boulders being 

 slowly cemented by a magnesio- calcareous paste derived from 

 the wasting beds of the great limestone series. For further 

 details regarding the natural history of the Dolomitic Conglo- 

 merate I must refer to a valuable memoir on this formation by 

 Mr. Etheridge, F.R.S.1 



2 he Rhcetic. — Singular beds of cherty and sandy deposits of 

 Rhactic age occur in several parts of the Mendips, in places brec- 

 ciated, or as a conglomerate, and resting either upon the Dolo- 

 mitic Conglomerate or Carboniferous Limestone. 



The fossils are either cherty, or they have been removed, and 

 iheir moulds are formed of chert, or cavities are left where 

 organisms existed. 



These beds are exposed at East Harptree, Egar Hill, Ashwick, 

 and Shepton-Mallet. In the Vallis they repose immediately on 

 the upturned edges of the Carboniferous Limestone, and even 

 fill in the numerous veins, pockets, and faults in that formation, 

 with fossil species common to the beds. 



Nowhere can the geologist read more clearly the physical 

 history of the groups of associated rocks composing the structure 

 of the Eastern Mendips than at Wells, the Vallis, Watley, Elm, 

 Nunny, and Holwell, where Old Red Sandstone, Carboniferous 

 Limestone, Coal Measures, Dolomitic Conglomerate, Rha;tic 

 beds, Lia.s, and Oolites are all exposed in natural sequence to 

 each other. There can be no doubt that the Rhaetic sea sur- 

 rounded and covered the Mendips ; for its remains are found 

 reposing on the Old Red Sandstone, Carboniferous Limestone. 

 Coal Measures, and Dolomitic Conglomerate, and pass upwards 

 into the Lias beds. 



The Lias. — Fragmentary portions of this formation are found 

 resting upon the summits of the Mendips, covering respectively 

 Old Red Sandstone, Carboniferous Limestone, Dolomitic Con- 

 glomerate, and Rhaetic beds, and in the Holcombe and Bar- 

 rington districts resting upon the Coal Measures, proving the 

 former extension of the Liassic sea over the Mendips ; for upon 

 some of their highest points, as near as Castle Comfort, the 

 cherty beds, with their characteristic fossils, are found ; also at 

 Chewton Mendip, Emberrow, Ashwick, &c. ; and on tlie south 

 side of the hills it is found at a considerable height, as at Dow- 

 side, Chilcott, and West Herrington. During the Lias age the 

 Mendips must either have been an archipelago, or they were 

 totally submerged beneath the sea which deposited the Liassic 

 plain to the north and south. The re-elevation of the Mendip 

 range has occasioned the removal by aqueous denudation of 

 most of the Lias beds deposited on their summit, whilst along 

 the southern flanks of the hills, and in the valley, a considerable 

 thickness of this formation still remains in siiu. 



Igneous Rocks. — Mr. Charles Moore * has shown that there is 



■ Quart. Journ. Gaol. Soc. vol. xxvi. p. 174 (1870). 

 - Ibid. vol. xxiii. p. 452 18O7). 



an exposure of basaltic rocks (dioritic) along the anticlinal of the 

 Mendips, a little west of Downhead, extending visibly nearly as 

 far as Beacon Hill, between two and three miles in length and a 

 quarter of a mile in width. 



This igneous mass appears in the form of a dyke, and is co- 

 incident with the anticlinal line along the axis of the Mendips, 

 which is here traceable for seven miles, and is again continued 

 from near Harptree to Shipham. 



There is likewise at the south end of Uphill cutting (Bristol 

 and Exeter Railway), at the western extremity of Bleadon Hill, 

 an extensive patch of igneous rock, discovered when that line 

 was made, and described by Mr. W. Sanders, F.R.S. ; this ex- 

 posure was also in the line of the anticlinal, and ended in the 

 fault which there crosses the line. This rock, according to Mr. 

 Rutley's analysis, is a Pitchstone Porphyry, whilst Mr. David 

 Forbes considers it a Dolerite. 



Whether this dyke was really eruptive or overflowed the Old 

 Red Sandstone is still a question to be solved ; and whether it 

 is co-extensive with the range is unknown ; but its age must be- 

 subsequent to the Coal Measures— the whole of the Palicozoic 

 rocks being disturbed alike, and lying at one general angle of 

 inclination, the overlying secondary strata not being influenced 

 or at all affected by these Palaeozoic changes. The Old Devonian 

 rocks in contact with the dyke are not altered or metamor- 

 phosed, thus establishing the facts of age and condition. 



3. The Radstock District. 



Among the many interesting features of the neighbourhood in 

 which we are assembled is the Bristol Coal-field, which still 

 offers an inexhaustible subject for scientific inquiry ; extending 

 from Cromhall in the north to Frome in the south, and from 

 Bath in the east to Nailsea in the west, comprising an area of 

 238 square miles. 



From a very early date it attracted the attention of geologists, 

 and was long ago the subject of a paper by Mr. Strachey, which 

 was published in one of the local societies. Dr. Buckland^ 

 contributed an able memoir on this Coal-field, in which a great 

 quantity of important information was placed on record, which 

 has been of the greatest possible use down to the present time. 



Subsequently this area has formed the subject of able papers 

 contributed to the North of England and South Wales Institutes 

 of Engineers, by Mr. J. C. Greenwell, F.G.S., and Mr. Handel 

 Cossham, F.G.S., and to other scientific .societies by Mr. Robert 

 Etheridge, F.R.S., and Mr. Charles Moore, F.G.S. 



During the past twelve years Mr. J. M'Murtrie, F.G.S., of 

 Radstock, has been continuously engaged in working out the 

 physical geology of the district, and has contributed a scries of 

 memoirs on the Bristol Coal-field to the Bath and Somerset- 

 shire Societies, which have thrown a new and important light 

 on these marvellous disturbances which have distorted the 

 strata. 



That part of the Report of the Royal Coal Commission bear- 

 mg upon the Bristol Coal-field, and prepared by Professor 

 Prestwich, and papers by Mr. Horace Woodward and Mr. John 

 Anstey, have summarised our previous knowledge, and added 

 recent facts thereto ; but with all that has been done much remains 

 to be investigated before a full history of the Bristol Coal-field 

 can be written. 



Although more or less connected throughout, the Coal-fields 

 adjoining Bristol consist of three well-defined areas, called the 

 Gloucestershire, Radstock, and Nailsea basins, each of which 

 has its own distinctive features. The Gloucestershire is sepa- 

 rated from the Radstock basin by the great Kingswood anticlinal, 

 which intersects in a ridge-like form the entire Coal-field from 

 east to west ; and the Nailsea basin has been almost, if not 

 entirely, cut off from the principal coal district by the elevated 

 limestones of Broadfield Down. Of these three areas Radstock 

 basin is the most extensive, both geographically and sectionally, 

 a great portion of its thickness being yet entirely undeveloped. 

 One of the features which will be remarked by visitors coming 

 from other parts of England is the number and character of the 

 Secondary formations by which the Radstock ba.sin is overlain. 

 Here and there, it is true, Mesozoic rocks have been denuded ; 

 but by far the greater portion of the Coal-field is hidden beneath 

 a covering of New Red Sandstone, Lias, and inferior Oolite, 

 and many of the shafts have had to pass through all these forma- 

 tions before the coal-seams were reached. 



A very slight change in the geological circumstances of the 

 past would have left us in entire ignorance of the cxi»tcnce of a 

 Coal-field so far south as Bristol ; and this reflection induces the 

 » Trans. Geol. Soc. Second Scries, vol. i. 



