DEPOSITS IN THE BRISTOL DISTRICT. 



79 



from the Carboniferous Limestone *." Looking at the Yate deposit as 

 it lies on the limestone in the section, it has much resemblance to the 

 Clevedon beds, but on closer examination this ceases. The latter 

 are almost pure Magnesian Limestone, whilst the upper 9 feet of 

 the former are of sandy yellow marls, which on being washed float 

 away and leave a sandy residue. Next follow eighteen inches of 

 yellow limestone, in two irregular beds, which in structure and 

 colour somewhat resemble the Clevedon stone. On treatment with 

 acid these also leave a sandy residue. Between the above and the 

 Carboniferous beds there is a thin deposit in pockets, almost com- 

 posed of fine grey sandstone. There are no organic remains special 

 to these beds beyond fucoids and Tubutella ambigua, which is rare. 

 For these reasons, and knowing how varied are the deposits on the 

 outcrops of the older limestones, I am disposed to think that the 

 yellow beds at Yate are of comparatively recent age. On the 

 surface of the Carboniferous Limestone there are occasional thin 

 patches of what appears to be a comminuted shelly breccia with a 

 grey sandy matrix, which I do not think the equivalent of the 

 bouldered Dolomitic Conglomerates found in other parts of the 

 district. 



Thornbury Railway and Secondary Veins. — The branch line which 

 leads from the Yate station to Thornbury exhibits some interesting 

 geology. At Tytherington fine sections of Carboniferous Limestone 

 are seen, and thick deposits follow of what are, no doubt, true 

 Dolomitic Conglomerates. On emerging from the tunnel towards 

 Thornbury these have some marly divisions, in which, in a flat over 

 the tunnel, some galena has been found. Not far beyond, the Lower 

 Limestone shales pass into the Upper Devonian beds, the conglo- 

 merates also resting upon them. Just before reaching this point 

 there are several thin veins in the limestone containing sulphate of 

 barytes and galena, in a matrix of gossany iron-ore. A sample 

 from the soft ochreous part yielded me three Conodonts of Carboni- 

 ferous-limestone age — and of (Secondary remains, Pentacrinite joints 

 and a single specimen of a Eoraminiferous shell, Planularia pau- 

 perata. On a second visit I discovered deposits towards the surface 

 which, lithologically, I cannot distinguish from a ferruginous marl 

 of the Middle Lias, which contains hollow casts of shells and crushed 

 specimens of what appear to be Wiynclionella tetrahedra. Tubutella 

 is present in great numbers, surrounded, as at Yate Bocks, by a re- 

 deposited ferruginous matrix. 



Age of the Deposits. — From the foregoing examples of abnormal 

 deposition, all of which have been accidentally revealed by quarrying- 



* Since writing the above, I have sent the specimens to my friend Mr. 

 Davidson, who says: — " The specimen you send for my examination you say is 

 from the Magnesian limestone. In colour it looks like a rock of that for- 

 mation ; but I have never hitherto seen from our British Permian rocks a Stro- 

 jphomena or Sfreptorhynchus shell similar to the one I observe on both sides of the 

 specimen, and which looks like Stro'phomena crenistria. The Ehynchonella is 

 not sufficiently complete for specific determination. If not Carboniferous, at 

 least one of the species would be new to our Permian rocks or Magnesian Lime- 

 stone. I almost fear your enclosed specimen is Carboniferous." 



