150 II. B. Woodward — Inversion of Coal Strata. 



folding-over of the main ridge or anticlinal of the Mendip range, 

 but no one, so far as I am aware, has been bold enough to repre- 

 sent the idea in diagram. 



The subject is to a great extent theoretical, — the facts connected 

 with it are not sufficient to render possible a definite solution of 

 it, — but the strata are so remarkably disturbed, that it could hardly 

 be otherwise. I have for some time been puzzled to draw a section 

 that would satisfactorily account for the phenomena, and being led 

 to differ from the prevalent notion, I venture to publish one, which 

 I think will explain the general structure of the district. Or, 

 perhaps, by exciting discussion, it may be the means of eliciting 

 further information, and of thus obtaining more correct information 

 upon the subject. 



At Luckington and Vobster, near Coleford, three patches of 

 Mountain Limestone appear in the midst of the Coal-measures. 

 When originally mapped by the officers of the Geological Survey,^ 

 they were considered to be due to upheaval, bosses of the rock being 

 represented in their Horizontal Sections as thrust up through the 

 Coal-measures.^ With the facts then observed, this was the safest 

 theory to adopt, but now that coal has been worked beneath the 

 Limestone a different theory is necessary to account fox their posi- 

 tion, and an inversion of some sort must be the correct explanation. 



Liversions of strata are by no means new features in this district. 

 In 1824: Messrs. Buckland and Conybeare pointed out that in the 

 Nettlebridge Valley the coal-strata are sometimes in a vertical 

 position, for at Pitcot a shaft was sunk in one bed of coal to a 

 depth of eighty fathoms, while towards Mells they noticed the 

 strata to be actually inverted, and occasionally so disturbed that at 

 Vobster the same bed of coal was thrice passed through in a sliaft.^ 



In reference to this subject, by far the most important contributions 

 are those of Mr. McMurtrie. 



In his paper on the " Faults and Contortions of the Somersetshire 

 Coal-field," he mentions the folded and contorted strata of the 

 Nettlebridge Valley, and he remarks that the amount of confusion 



are in some instances folded over upon themselves. In illustration of this he men- 

 tions the working of coal beneath the Mountain Limestone of Luckington. 



J. McMurtrie, "Faults and Contortions of the Somereetshire Coal-field," a paper 

 read before the Bath Nat. Hist, and Antiq. Field Club, 24th Feb., 1869. 



Mr. S. W. Brice, in his "Essay on the Coal-field of North Somersetshire, " 1867, 

 mentions that at Vobster branches have been driven right under the Mountain 

 Limestone in search of coal. And he suggests that when the Mendip range was 

 considerably higher than it is now, masses of the Limestone might easily have been 

 dislocated from their parent rock, and have rolled down a species of inclined plane 

 into the positions they now occupy. 



1 Two of these, the one at Luckington and the other at Upper Vobster, were 

 laid down on the Geological Survey Map, sheet 19. During a re-survey of the 

 ground, Mr. W. A. E. Ussher has carefully traced out the boundaries of the three. 

 They were very accurately laid down by Mr. W. Sanders, F.R.S., in his " Map of the 

 Bristol Coal Fields, and country adjacent," 1864. 



2 A similar section was given by Buckland and Conybeare. Trans. Geol. Soc, 

 series 2, vol. i. 



3 Trans. Geol. Soc, series 2, vol. i., p. 255. See also Conybeare and Phillips, 

 " Geology of England and Wales," 1822, p. 428. 



