154 H. B. Woodivard — Inversion of Coal 8trata. 



The relative thicknesses of the formations, and their general dip, 

 have of course received attention. 



The fan-shaped anticlinal would have exerted sufficient force to 

 produce any amount of contortion in the Coal-measure shales ; the 

 twisted beds to the south and the inverted strata on the north are 

 well accounted for. The beds of course would not have stretched 

 across the entire distance represented by the dotted line ; great rents 

 and fractures must have occurred, and while the beds, as is well 

 known, were very much faulted, there is not much difficulty in ima- 

 gining the little patch of Limestone at Lower Vobster to be the relic 

 of a mass faulted into such a position — along with the Coal-measures 

 which have been let down between the masses of Limestone.^ 



In regard to the origin of the Limestone at Luckington, a similar 

 and probable continuation of the same disturbance would account 

 for it. As " reversed " faulting has been introduced, it should be 

 mentioned that cases of this are not uncommon in the Mendip area. 

 In the last volume of the Geological Magazine attention was re- 

 directed to a reversed fault at Uphill, near Weston-super-Mare, while 

 in the Eadstock Coal-field there are numerous "overlap faults," as they 

 are termed, where seams of coal have been forced over one another, 

 so that pits sunk on the spot pass twice through the same seams.^ 



The Millstone Grit has been inserted conformable to the Moun- 

 tain Limestone on its northern side at Upper Vobster. This is 

 purely hypothetical, for the surface is obscured by a covering of 

 Inferior Oolite. As the beds are known to be inverted to the north, 

 the Grit might very well come in between the Coal-measures and 

 the Mountain Limestone. The explanation at any rate is as satis- 

 factory as one that would cut it off again with the aid of another fault. 



And now as regards the plan : two at least of the patches must 

 be surrounded by faults. But considering the great disturbances 

 to which the beds have been subjected, the dislocations must have 

 been very numerous. Faults are known to be common ; on the 

 original theory the bosses of Limestone were surrounded by faults ; 

 therefore one need not be scrupulous about retaining them. 



Of several varieties of structure I have drawn, the one now 

 given appears to me the most satisfactory. The problem is as full 

 of difficulty as it is of interest. It seems to me that the isolated 

 patches of Limestone could not have been thrown over from the 

 main anticlinal ; the explanation I bave drawn seems to accord with 

 the known facts. And here I leave it, in the hopes that it may 

 tend, in some degree, to the elucidation of the phenomena. 



1 This is the kind of structure so characteristic of the Alps — as noticed by Studer, 

 Desor, Lory, and others. 



2 G. C. Grreenwell, " Notes on the Coal-field of East Somerset." Trans. North of 

 England Inst, of Mining Engineers, vol. ii., 1854, p. 255. * 



Greenwell and McMurtrie, " On the Eadstock portion of the Somersetshire Coal- 

 field." 1864. p. 17. 



McMurtrie, " Faults and Contortions," etc. 



The faults in the Somersetshire Coal-field are worthy of much study from their 

 variety and remarkable characters. Mr. McMurtrie mentions an instance of beds thin- 

 ning out towards a fault, a fact that may sometimes be noticed in the Lias where it 

 is similarly afifected. 



