354 



NATURE 



\_Aug. 26, 1875 



hope that in other parts of the country (at present believed to 

 be without coal, or, if present, to lie at such a depth from the 

 surface that it cannot be worked) it may yet be discovered at a 

 moderate depth. 



Another feature of the Radstock Coal Measures is their great 

 thickness, which Mr. M'Murtrie estimates at 8,000 feet. From 

 this we may infer that, however limited the area in Somerset- 

 shire of which we have at present positive knowledge, we are 

 very far indeed from the edge of that infinitely more extensive 

 area which the Coal Measures of the South of England originally 

 occupied, and within which outlying basins may still be found. 



It is abundantly evident that the Bristol Coal-field was origin- 

 ally connected with that of the Forest of Dean and South Wales, 

 with which it has many characters in common, although it differs 

 in other respects.' 



In all we find the same arrangement of the different strata, 

 namely : — ist, an upper division of productive Coal Measures ; 

 2nd, a central mass of Pennant Sandstone ; and, 3rd, beneath, 

 a lower division of productive Coal Measures resting upon, 4th, 

 the Millstone Grit. Hitherto it has been found impossiljle to 

 correlate the seams of coal ; but they present many points of 

 general correspondence in the districts referred to ; and the in- 

 formation obtained leads to the conclusion that their greatest 

 sectional development occurs between Radstock and Bristol, 

 according to the following estimate of the thickness of the strata, 

 number of seams, and thickness of coal-seams : — 



Table II. — Strata and Coal-Seams. 



This great sectional thickness is attended, however, with 

 serious disadvantages ; for although, according to the Report of 

 the Royal Coal Commission, the Bristol Coal-field was estimated 

 to contain 6, 104 millions of tons of coal, a large portion of it 

 lies at an unworkable depth. Another physical feature of the 

 district is the thinness of many of the seams from which coal is 

 at present obtained. 



In many of the collieries seams of from 10 to 12 inches in 

 thickness are extensively worked, thus setting a good example 

 of economy of one of our most precious natural productions to 

 other parts of England, where veins of similar thickness are left 

 behind as worthless. 



Another feature of the Radstock Coal-basin is the extreme 

 richness of its beds in the fossU flora of the Coal Measures. The 

 Pennant Sandstone and Lower Measures yield few plants ; but 

 the Upper divisions contain much finer specimens than I have 

 seen elsewhere, and the fossil flora of Radstock preserved in Mr. 

 M'Murtrie's museum is alone worth a journey to study and 

 admire. The fossil ferns are in great variety and beautifully pre- 

 served. The Sigillaricc, Lepidodendra, and other acrogenous 

 stems tell of the arborescent ferns that floated their plume-like 

 foliage on the islands of the Carboniferous period, and the 

 industry and genius of the man who has collected and preserved 

 them for our instruction and delight. The animal remains are 

 here very scarce ; two or three species of the genus Limulus, 

 and one or two Anthracosio:, are all that have been found ; and 

 I have the satisfaction of adding that I am authorised to say 

 that by previous arrangement Mr. M'Murtrie will be happy to 

 show his museum to any members of the Association to whom 

 the same might be interesting. As there will be, I understand, 

 memoirs on the Radstock Coal-field, I must refer to these papers 

 for further details on this interesting district. 



4. The Bristol District. 

 In a radius of eight miles from the Guildhall we find ex- 

 posures more or less complete of the following Palseozoic and 



Mesozoic formations : — i. The Old Red Sandstone ; 2. the Car- 

 bonifet ous Limestone ; 3. Millstone Grit ; 4. Coal Measures ; 5. 

 Dolomitie Conglomerate and Nexv Red Sandstone ; 6. Rhatic ; 7. 

 Lias, Lower, Middle, Upper ; 8. Upper JJas Sands ; 9. Inferior 

 Oolite; 10. Fuller's Earth; II. Great Oolite ; 12. Alluvium, 

 with igneous rocks of Palaeozoic age. Several of these forma- 

 tions I have already noticed in speaking of the Mendip Hills ; 

 therefore I shall only now add such special remarks as are 

 required to complete their sketch in the Bristol district. 



The Old Red Sandstone forms, as we have seen, the axis of the 

 Mendip Hills, and here occurs as a massive rock in different 

 regions of the Bristol Coal-field, forming ranges of hills that 

 have been sculptured by denudation out of its anticlinal folds. 

 The beds in general are very unfossiliferous. 



In the neighbourhood of Portishead, however, the remains of 

 some large fishes have been found in a hard conglomerate, be- 

 longing to the genus Holoptychius — reminding us of the fishes of 

 the Old Red Sandstone of Scotland, which were all encased in 

 a bony armour, and possessed some of the most remarkable 

 forms of the ichthyic type. Pterichthys or wing-fish, Holoptychius 

 or wrinkle-scaled fish, Cephalaspis or buckler-shielded fish, are 

 all forms of the OKI Red, and the earliest representatives of the 

 class Pisces in the Paloeozoic rocks. 



The Carboniferous Limestone is a great marine formation, 

 and is formed of the sediments of an extensive and wide-spread- 

 ing sea ; the beautiful scenery so characterisric of the Avon, 

 Severn, and Wye is in a great measure due to the development 

 of this rock in these regions. One of the grandest sections of all 

 the beds of the Carboniferous Limestone is that exposed in the 

 gorge of the Avon near Clifton, where it is seen resting on the 

 Old Red Sandstone, and overlain by the Millstone Grit. 



The various conditions of the old sea-bottom in which this 

 mass of calcareous rock was formed may here be studied with 

 ease. The entire thickness of the strata exposed is upwards of 

 4,coo feet ; of this the Old Red Devonian is 768 feet, the Car- 

 boniferous Limestone 2,338, and the Millstone Grit 950 feet. 

 This magnificent section has repeatedly been the subject of 

 memoirs by Buckland,^ Conybeare,^ Bright,^ and Williams,'' 

 who have given ample details of all its different beds. 



The Lower Limestone Shales, 500 feet in thickness, are very 

 fossiliferous ; they consist of alternations of shales and lime- 

 stone, with a bone-bed near their base _: in some places beds 

 several feet thick are formed of the ossicula of Crinoids. I.i the 

 main Limestone series you I ave a succession of Brachiopoda ; 

 Spirifera, Producta, and Orthis follow each other. Of Lamelli- 

 branchs we find Aviculopecten, Cardioviorpha, Sec, with Gas- 

 teropods, as Euot>iphalus and BelUrophon, and Cephalopods, as 

 Goniatites, Orthoceras, Actinoceras, &c. To these may be added 

 the teeth and defensive spines of large shark-like and other 

 fishes, as Cladodus, Psainmodus, Orodus, IPoloptychius, &c. 

 Some of the coral strata in the upper part of the series are very 

 interesting, and extremely rich in very beautiful specimens of 

 Actinozoa, belonging to the reef-building groupsof the ancient sea, 

 as Michelinia, Anipiexus, Lithostrolion, Syringopora, Lonsdaleia, 

 &c., reminding us of the structure of coral reefs in our present 

 seas. Associated with the coral masses are other organisms 

 which lived on the reefs, or in .shallow lagoons. The coral beds 

 are covered by strata formed of Oolitic limestone and other 

 detrital materials derived from the debris of wasted reefs, and 

 formed along the shores of the ancient coral strand ; sections of 

 these oolitic beds prepared as slides for the microscope disclose 

 the fact that the nucleus of the oolitic granules is often the shell 

 of Foraminifera. 



Milhtons Grit is well seen at Brandon Hill ; it rests upon the 

 Limestone, and attains a thickness of 1,000 feet. On this 

 repose the Coal Measures of the Bristol Coal-field, which I have 

 already described in connec.tion with the Mendip and Radstock 

 districts. 



Dolomitie Conglomerate. — The Paloeozoic rocks of the Bristol 

 Coal-field are here and there covered over by patches of Dolo- 

 mitie Conglomerate lying unconformably on their upturned edges, 

 at heights varying from 20 to 300 feet above the Avon. This 

 remarkable formation is very well seen in the new road leading 

 from the Hot- wells to Clifton and Durdham Down. It has been 



I " On the South-Eastem Coal District of England," Geol. Trans. 2nd 

 series, vol. i. 



^ Geol. Trans, ist series, vol. iv. 



3 "On the Limestone Beds of the River Avon," Geol. Trans, ist series, 

 vol. v. 



** " Memoirs of the Geol. Survey," Sir H. De la Beche's Essay, vol. i. 

 p. 115. 



