854 * REPORT — 1898. 



Bridgewater flats, a change is already noted in the case of the Cannington Park 

 limestone, which was the subject of so much discussion m former years. Referring 

 to this, Mr. Handel Cossham ' 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 limestones, is alreadj' iu full force. 



In the new ' Index-map ' the Culm-measures are placed at the base of the Car- 

 boniferous series — below the Carboniferous Limestone. It is no part of my purpose 

 to attempt unj precise correlation, but I would point out the somewhat singular 

 circumstance 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 

 Ked 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 Radiolarian rocks. One result of the im- 

 portant paper of Messrs. Ilinde and Fox has been to alter materially our views 

 as to the physical conditions accompanying the deposition of a portion of the 

 Oulm-measui-es. The palfeontology leads the authors to conclude ^ that ' the 

 Lower Posidonomya- and Waddou Barton Beds are the representatives and equiva- 

 lents of the Carboniferous Limestone iu other portions of the British Isles ; not, 

 however, in the at present generally imderstood 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 representatives of the shallower-formed 

 calcareous deposits to the north of them. . . . The jncture 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 Radiolarian rocks.' 



This is probably a correct view of the case ; but one cannot help wondering 

 that the ocean cuvreuts and other causes did not eli'ect a greater amount of com- 

 mingling of the elements than seems to have taken place. As a practical result, 

 this discovery of a Radiolarian horizon in the Culm-measures has been of 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 when the paper was read before the Geological Society. 



The principal features of the Bristol coal-field are too well known ito call for 

 many remarks. It would seem that the Pennant 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.'' It might seem almost unnecessarj' to refer to the 

 existence of sucli 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 

 Palfeozoics, in one great geosynclinal depression, to an entirely different class of 

 phenomena. Nowhere, perhaps, are the effects of the post-Carboniferous interval 

 better exhibited than in those parts of the South-west of England where Tertiary 

 denudation has removed the Mesozoic deposits. Here we perceive some of the 

 effects of the great foliations which terminated the Pala30zoic epoch in this part 



' Froo. Cottcs. Club, vol. viii. (1881-2) p. 20 et setj. 



2 Quart. Jour7i. Geol. tioc. vol. li. (1895), p. 662. 



' Since the Address was read, I have found that what 3Ir. Cossham showed was 

 that a small tract marked 'Millstone Grit ' in the survey map at Kingswood, east 

 of Bristol, belongs to the Pennant Grit (see Gcol. Maq. ii. 1 10). 



' Froc. Cottes. Clvb, vol. vii. (1878), p. 73. 



