Carboniferous Limestone at Upper Vobster, 207 



Somerset Coalfield, have been the subject of comment and specula- 

 tion by many writers since two of them were" first noticed by 

 Auckland & Conybeare. 



The Upper Vobster 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 1100 yards. 



From personal observation, combined with important evidence 

 recorded by previous investigators, the 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 over- 

 folded strata of the Coal Measures by powerful thrust- 

 movements. 



(2) It comprises (a) a Northern Limestone Mass, and (b) 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 Dibunopliyllum 

 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 Dibunopliyllum Zone and the Upper Seminula Zone 

 are exposed, the strata are locally overfolded northwards. 



(G) 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. 



The occurrence of a lamellibranch-fauna at the top of the 

 Seminula Zone is recorded as a feature of special interest. 



