Vol. 51.] TERTIARY DATE IN DORSEI. 557 



numerous Oxford Clay fossils l and some blocks of Combrash. The 

 A dyke ' where crossed by the cutting is about 20 or 30 yards wide, 

 and was followed down in shafts to a depth of 50 feet, where it still 

 •kept the same character, and was still cut off northwards by a clean, 

 nearly vertical wall of Chalk. 2 Between the middle part of the 

 Wealden group and the top of the Oxford Clay, there would, in a 

 normal sequence, be a thickness of about 2000 feet of strata. 



The explanation hitherto given of the appearance of the Oxford 

 Clay at Bidgeway depended on the existence of the two systems of 

 disturbance to which I have already referred. Mr. Hudleston 

 {remarks : — ' The most probable supposition is that there exists a 

 double fault here, and that one of the faults is of pre-Cretaceous 

 age, pointing to extensive pre-Cretaceous denudation. Thus the 

 base of the Upper Cretaceous (here the Upper Greensand) at 

 Bidgway may really rest upon Oxford Clay, as is represented to be 

 the case at the inlier of East Compton, a few miles to the north- 

 west. In this case the Wealden-Purbecks would not pass under 

 the Chalk, as they are bound to do, if this were a simple fault 

 hading to the downthrow in the ordinary manner. Thus the old 

 line of disturbance seems to have become a line of weakness in 

 post-Cretaceous times, although the direction of the throw must 

 fiave been reversed.' 3 



It seemed in favour of this explanation that a. fault at Abbots- 

 bury, with a large downthrow south and belonging to the earlier 

 group of disturbances, does actually fall into nearly the required 

 ■direction. By supposing that this fault ran on about 6 miles west- 

 ward, nearly along the line of the Bidgeway Fault, but crossed here 

 and there by it, the structure appeared intelligible. On further 

 consideration, however, difficulties arose which I had not at first 

 realized. In the first place, it was necessary to imagine that a 

 post-Cretaceous northerly downthrow was superimposed almost 

 exactly for several miles on the earlier southerly downthrow ; 

 <next, we had to suppose that this earlier fault had a throw of 

 2000 feet, and was therefore far larger than any other fault in the 

 Secondary rocks of the South of England. Finally, to account for 

 the Bincombe inlier this great fault had to make four sharp turns, 

 intruding what could only be compared to a pyramid of Oxford 

 Clay and Cornbrash into the Kimmeridge Clay, an almost incon- 

 ceivable structure. It is, in fact, impossible to devise a combi- 

 nation of normal faults which will account for the position of that 

 inlier without involving great improbabilities. 



The alternative explanation is that the faults are of all the same 

 post-Cretaceous age, and that they are overthrusts. Thus the 

 'Oxford Clay of Bidgeway has not been faulted against the Wealdeu 



1 Mr. H. B. Woodward, in a forthcoming Memoir on the Middle and Upper 

 Oolites, points out that the fossils belong to the middle and upper portions of 

 the Oxford Clay, although they occurred in proximity to blocks of Cornbrash. 



2 The shafts seem to have been sunk from the former surface of the ground 

 to the level of the rails, not 50 feet below the rails. 



3 Report on the Easter Excursion, 1889. Proc. Geol. Assoc, vol. xi. p. lii. 



