152 H, B. Woodivard — Inversion of Coal Strata. 



exposed from Mells to near Ashwick ; it was probably reached in a 

 boring at Cbewton Mendip ; ^ Mr. Sanders has noticed a trace of it 

 near Compton Martin ;^ and in the summer of 1869, 1 mapped traces 

 of it near Wells, on the southern slope of the Mendips, where, 

 however, it had previously been observed by Mr. "Williams, of the 

 Geological Survey. Being unaware of this at the time, I was much 

 pleased to find my work corroborated.^ 



Having come to the conclusion that the Millstone Grit has not 

 died out, we have still to account for its absence, and the aid of 

 faults must be invoked in order to get rid of it. 



But are there any facts to show that the main ridge of Mendip was 

 folded over ? Now, although the Coal-strata present every variety 

 of disturbance, this is decidedly not the case with the Mountain 

 Limestone and Old Eed Sandstone. On the northern side of the 

 Mendips they attain a much higher general inclination than on the 

 southern side, and in one locality (near Stoke Lane) Mr. Sanders has 

 observed the Limestone in a vertical position. But never, so far as 

 I am aware, has it been noticed to be inverted along the borders of 

 the Nettlebridge Valley, the average dip being between 60° and 70° 

 in a northerly direction.'' 



The Coal-measures must not be taken as affording a type of the 

 amount of disturbance which the district has suffered — the whole 

 series of rocks would be bent and folded, but whilst under pressure 

 the Mountain Limestone and Old Eed Sandstone would be hard and 

 unyielding, the coal-shales would be squeezed up and contorted in 

 all manner of ways, as, indeed, they are now found.® 



Mr. McMurtrie remarks that the "New Eock series," being chiefly 

 composed of strong sandstones, does not present so much distortion 

 as the other beds in the Coal-measures. 



There seems to be no evidence in favour of the theory that the 

 Mendip anticlinal was folded over, and the facts observed are against 

 such a notion. 



By introducing a fold further to the north, and by calling faults 

 to our assistance, the position of these Limestone masses may, I 

 think, be satisfactorily accounted for — and without resorting to any 

 form of structure which is not already known in the Mendip area. 



The accompanying section shows my attempt to explain the 

 general structure. 



1 McMurtrie — Lecture on the Carboniferous Strata of Somersetshire, 1868, p. 14. 



2 Map of the Bristol Coal Fields, etc. 



3 De la Beche, Mem. Geol. Survey, vol. i., 1846, p. 413. 



* Vide Sanders' Map. The only locality where anything like an inversion has been 

 observed, is by the road-side at Churchill Batch, at the western edge of the Mendip 

 range. Proceeding along the high road in a northerly direction, the Mountain 

 Limestone is seen dipping at a high angle to the north, then it becomes vertical, and 

 a little further on the beds dip to the south. The whole is seen clearly in one section, 

 and it forms an excellent example of " fan-shaped " structure. This was originally 

 noticed by Buckland and Conybeare, who regarded it as a sharp synclinal. Mr. 

 McMurtrie admits that the Mountain Limestone has not as a rule been folded back, 

 regarding the Vobster patches as exceptions. 



® I find that the same idea is expressed by Conybeare and Phillips, op. cit., pp. 347 

 and 360. 



