September 15, 1898] 



NATURE 



ATT 



very youngest rcx;ks ; while east of the Severn and south of the 

 Bristol Channel true Boulder clay is rare or absent. 



It may be convenient to consider a few points which have 

 arisen of late years in connection with the geology of portions of 

 the district now under consideration. 



PalcEozoic. — If we omit the Silurian inlier at Tortworlh, the 

 geological history of the country, more immediately round 

 Bristol, may be said to commence with the Old Red Sandstone, 

 whose relations with the Devonian towards the south-west, have 

 always presented some difficulty. And this difficulty is accentu- 

 ated by doubts as to the true Devonian sequence in West 

 Somerset and North Devon. Ever since the days of Jukes 

 that region has been fruitful in what I must continue to regard 

 as heresy until the objectors have really established the points 

 for which they are contending. The uncertainty is to be re- 

 gretted, since it is through these beds of West Somerset that 

 the system is to be made to fit in with the several members of 

 the Old Red Sandstone. 



There is a mystery underlying the great alluvial flats of 

 Bridgewater which affects more than one formation ; so much 

 so, that one cannot avoid asking why there should be Old Red 

 Sandstone in the Mendips and Devonian in the Quantocks. 

 The line which separates the Old Red Sandstone of South 

 Wales and the Mendips from the West Somerset type of 

 Devonian lies here concealed. I have already suggested 

 {Trans. Devoush. Assoc, vol. xxi., 1889, p. 45) that, if we 

 regard the Old Red Sandstone of South Wales as an inshore 

 deposit over an area which was deluged with fresh water off 

 the land, we can believe that further out to sea, in a south- 

 westerly direction, the conditions were favourable for the de- 

 velopment of a moderate amount of marine mollusca. This 

 view not only does away with the necessity for a barrier, but 

 it also, in a general sense, suggests a kind of gradation be- 

 tween the Old Red and Devonian deposits. Mr. Ussher, 

 whose practical acquaintance with this region dates from a 

 long period, stated a few years ago that, "As far as Great 

 Britain is concerned, the true connections of the Old Red 

 Sandstone beds with their marine Devonian equivalents have 

 yet to be carefully worked out on the ground."' I am not 

 aware that further progress has been made in this direction. 



The Carboniferous Limestone of the Bristol area has attracted 

 the attention of so many distinguished geologists that its palaeont- 

 ology and general features are tolerably familiar. Of late years 

 we owe some interesting petrographic details to Mr. Wethered. 

 The varying thickness of the Carboniferous Limestone and also 

 of the Millstone Grit in this part of England is noteworthy. If 

 we follow the Carboniferous Limestone in a south- westerly 

 direction, across the mysterious Bridgewater flats, a change is 

 already noted in the case of the Cannington Park limestone, 

 which was the subject of so much discussion in former years. 

 Referring to this, Mr. Handel Cossham {Proc. Cot(es. Club, 

 vol. viii , 1881-2, p. 20 et seq.) was so sanguine as to believe 

 that its identification with the Carboniferous Limestone would 

 have the effect of extending the Bristol coal-field thirteen miles 

 south of the Mendips. However this may be, all further traces 

 of Carboniferous rocks fail at this point. After crossing the vale 

 of Taunton, when next we meet with them in the Bampton 

 district, the Culm-measure type, with its peculiar basal lime- 

 stones, is already in full force. 



In the new " Index-map" the Culm-measures are placed at 

 the base of the Carboniferous series — below the Carboniferous 

 Limestone. It is no part of my purpose to attempt any precise 

 correlation, but I would point out the somewhat singular circum- 

 stance that the change to Culm rock occurs only a few miles to 

 the south-west of the line where, in the previous system, we 

 have already seen that the Old Red Sandstone changes into the 

 Devonian. This curious coincidence may be wholly accidental, 

 or it may be the result of some physical feature now concealed 

 by overlying formations. 



Since 1895 a new light has been thrown on the Lower Culm- 

 measures by the discovery of a well marked horizon of Radio- 

 larian rocks. One result of the important paper of Messrs. 

 Ilinde and Fox has been to alter materially our views as to the 

 physical conditions accomp.tnying the deposition of a portion of 

 the Culm-measures. The palaeontology leads the authors to 

 conclude {Quart. Journ. Geol. Soc. vol. li., 1895, p. 662) that 

 " the Lower Posidonomya- and Waddon Barton Beds are the 

 representatives and equivalents of the Carboniferous Limestone 



1 Prospects of obtaining coal by boring south of the Mendips, Proc. Som. 

 Nat. Soc, vol. xxxvi. (iSgOpt. 2, p. 104. 



in other portions of the British Isles ; not, however, in the at 

 present generally understood sense that they are a shallow- water 

 facies of the presumed deeper-water Carboniferous Limestones, 

 but altogether the reverse, that they are the deep-water repre- 

 sentatives of the shallower-formed calcareous deposits to the 

 north of them. . . . The picture that we [Messrs. Hinde and 

 Fox] can now draw of this period is that while the massive 

 deposits of the Carboniferous Limestone — formed of the skeletons 

 of calcareous organisms — were in the process of growth in the 

 seas to the north \i.e. in the Mendip area and elsewhere] there 

 existed to the south-west a deeper ocean in which silicious 

 organisms predominated and formed these silicious Radiolariarv 

 rocks." 



This is probably a correct view of the case, but one cannot 

 help wondering that the ocean currents and other causes did not 

 effect a greater amount of commingling of the elements thai> 

 seems to have taken place. As a practical result, this discovery 

 of a Radiolarian horizon in the Culm-measures has been oi 

 service in enabling surveyors to discriminate between Devonian, 

 and Carboniferous in the very obscure area on the other side of 

 Dartmoor. This, I ventured to predict, would be the case whenr 

 the paper was read before the Geological Society. 



The principal features of the Bristol coal-field are too welb 

 known to call for many remarks. It would seem that the Pen- 

 nant rock was formerly regarded as Millstone Grit, until Mr.. 

 Handel Cossham, in 1864, pointed out the mistake. Mr. 

 Wethered gave a good description of the Pennant in his paper 

 on the Fossil Flora of the Bristol coal-field (/P/w. Cottes. Club, 

 vol. vii , 1878, p. 73). It might seem almost unnecessary to 

 refer to the existence of such a well-known formation as the 

 Pennant, but for the fact that in a recent scheme of the 

 Carboniferous sequence in Somersetshire the Pennant rock was 

 wholly omitted. 



The interest now shifts from the almost continuous deposition 

 of the later Palaeozoics, in one great geosynclinal depression, to 

 an entirely different class of phenomena. Nowhere, perhaps, 

 are the effects of the post-Carboniferous interval belter exhibited 

 than in those parts of the south-west of England where Tertiary- 

 denudation has removed the Mesozoic deposits. Here we per- 

 ceive some of the effects of the great foliations which termi- 

 nated the Palaeozoic epoch in this part of the world. The 

 immense amount of marine denudation which characterises- 

 this stage is particularly obvious in the anticlinals, which 

 were the first to suffer, as they came under the planing action. 

 of the sea. 



Attention may be drawn to a peculiarity which has no doubt 

 been observed by many persons who have studied a map of the 

 Bristol and Somerset coal-field. It will be seen that the strike 

 of the Coal-measures is widely different on either side of a line 

 which may be drawn through Mangotsfield to a point north of 

 Bristol. The beds north of this line have for the most part a 

 meridional strike, nearly parallel with the present Cotteswold 

 escarpment ; south of this line the strike is mainly east and 

 west, though much curved in the neighbourhood of Radstock. 

 and the flanks of the Mendips. Of course this is only part of an- 

 extensive change in the direction of flexure, much of which is 

 still hidden under Mesozoic rocks. Mr. Ussher, in the paper 

 previously quoted, tells us that the line of change of strike may 

 be traced in the general mass of the Palaeozoic rocks, from near 

 Brecon in South Wales to the neighbourhood of Frome. This 

 means that within the Bristol district two distinct systems of 

 flexure must have impinged on each other in post-Carboniferous 

 times. Have we not here, then, another instance of extra- 

 ordinary change within the limits of our area ? This time it is. 

 not a mere change in the nature of a deposit, like that of the 

 Old Red Sandstone into the Devonian, or of the Carboniferous 

 Limestone into the Culm-rock, but a change in the direction of 

 the elevatory forces, which had made its mark on the structure 

 of our island even at that early date. 



At this point I ought to quit the Palaeozoics ; but there is just 

 one subject of interest which claims a momentar>' attention^ 

 viz. the probability of finding workable coal east of the proved 

 Somersetshire field. I avoid the question of coal south of the 

 Mendips as being too speculative, on account of the chances of 

 deterioration of the coal-measures in that direction. But in view 

 of the forthcoming meeting of the British Association at Dover, 

 the question of finding coal to the eastward of Bath becomes a 

 specially interesting subject for discussion. It is also a matter 

 of some consequence whether the hidden basin or basins belong 

 to the meridional or to the east and west system of flexures. 



NO. 1507, VOL. 58] 



