Reports and Proceedings — Geological Society of London. 45 



Decemier 6, 1911.— Professor W. W. Watts, Sc.D., LL.D., M.Sc, 

 F.R.S., President, in the Chair. 



The following communications were read: — 



1. "The Faulted Inlier of Carboniferous Limestone at Upper 

 Vobster (Somerset)." By Thomas Franklin Sibly, D.Sc, F.G.S. 



The three small masses of Carboniferous Limestone at Luckington, 

 Upper Yobster, and Tor Hock, Vobster, respectively, which lie in the 

 overfolded Coal-measures of the southern portion of the East Somerset 

 Coal-field, have been the subject of comment and speculation by many 

 writers since two of them were first noticed by Euckland & Conybeare. 



The Upper Yobster inlier is by far the largest of the three. 

 Intermediate in position, it lies rather less than a mile to the north 

 of the main outcrop of the Carboniferous Limestone of the Eastern 

 Mendips. This inlier has been dissected to a remarkable extent by 

 quarrying operations, and its structure is described in detail in the 

 present paper. 



The northern and eastern portions of the inlier are concealed by 

 a thin covering of Lias, but its maximum width from north to 

 south is probably little, if at all, greater than 400 yards, while the 

 proved east-and-west extent of the Carboniferous Limestone is about 

 1,100 yards. 



From personal observation, combined with important evidence 

 recorded by previous investigators, tlie author has arrived at the 

 following general conclusions : — 



(1) The inlier is apparently a lenticular mass of Carboniferous 

 Limestone, grits, and shales, superimposed upon the overfolded strata 

 of the Coal-measures by powerful thrust movements. 



(2) It comprises (a) a Northern Limestone Mass and (3) a Southern 

 Limestone Mass, separated by (c) a Grit-and-Shale Mass. 



(3) The beds of the Grit-and-Shale Mass are in faulted relation to 

 the Carboniferous Limestone on both sides. On the northern side 

 the immediately adjacent beds of limestone represent part of the 

 Seminula Zone ; on the southern side the adjacent beds belong to 

 the Lower Bihmophyllum Zone. On both sides the limestones are 

 locally distorted. 



(4) In the Northern Limestone Mass, Yobster Quarry exposes over 

 500 feet of Seminula Beds, overfolded towards the north-west, and 

 dipping south-eastwards at an angle of about 135°. 



(5) In the Southern Limestone Mass, where portions of the Lower 

 Bihunophyllum Zone and the Upper Seminula Zone are exposed, the 

 strata are locally overfolded northwards. 



(6) The beds of the Grit-and-Shale Mass comprise quartzites which 

 must certainly be assigned to the Millstone Grit. They also include 

 shales, with intercalated fine-grained sandstones, of considerable 

 thickness. Possibly this mass includes the lowest beds of the Coal- 

 measures, in addition to a portion of the Millstone Grit. 



(7) In most of the sections of Carboniferous Limestone, signs of 

 the immense stresses to which the strata have been subjected are very 

 evident. The beds are often distorted, while slickensides and calcite 

 veins are extensively developed on both a large and a small scale. 



