118 REPORT— 1900. 



with numerous brownish -grey flints ; the Chalk being fissured but not 

 altered. At Station NN the hole was 7 feet deep and exhibited Chalk 

 with numerous flints, the rock being much slickensided and fissured. It 

 contained a few fragments of Inoceramus. I was not able anywhere to 

 get a satisfactory dip in the Chalk in the trench or holes ; though the 

 general impression suggested was of an ascending succession northward, 

 and of a high dip in that direction. 



The general results of the geological examination may thus be sum- 

 marised. The fault, at the point where the apparatus crosses it, probably 

 cuts out strata having a thickness of nearly 1,000 feet, made up thus : — 



Chalk (part of Upper, whole of Middle and Lower) . . . ,300 



Greensand and Gault 150 



Wealden 350 



Upper Purbcck 50 



Middle Purbeck 50 



Lower Purbeck (to within 5 feet of base) 85 



Total feet 985 



The break, however, is not caused by a normal fault of 985 feet throw. 

 It is the result of a sliding movement over a cylindrical surface curving 

 downward and northward from nearly vertical to nearly horizontal. 

 This view, as pointed out by Mr. Strahan, explains the presence of a 

 dyke of Oxford Clay and Cornbrash in the railway-cutting ; a fact which 

 cannot be satisfactorily accounted for by normal faulting, even to the 

 extent of 2,500 or 3,000 feet. The movement along the curve of the 

 thrust-plane amounts to not less than 2,500 feet, even if the strata are 

 everywhere vertical to the fault. It is just possible, however, that earlier 

 faulting along nearly the same line in intra- Cretaceous times brought up 

 Cornbrash, so that it occurs immediately beneath the Upper Cretaceous 

 rocks just north of the Tertiary fault. On this supposition, and with 

 the most favourable angle of dip throughout, the Tertiary thrust may not 

 exceed 500 feet. The most probable estimate of the extent of the 

 Tertiary displacement is, however, about half a mile ; a lower estimate 

 demands an improbable series of fortuitous coincidences, such as we are 

 not j ustified in postulating. 



There is one point that I should like to suggest for future considera- 

 tion. The disturbances just described result from lateral compression of 

 the strata in a north and south direction, and it is clear that levelling 

 across the fractures will only give us one element in that motion. The 

 horizontal movement must be of much greater magnitude than the vertical, 

 and could be accurately tested by triangulation. As the folds have always 

 an east and west axis, and there is no sign of disturbance in other 

 directions, triangulation across the folds from fixed points lying east and 

 west ought to enable us to test whether any change is now going on over 

 wider areas. Even a comparison of the earlier Ordnance triangulation 

 of the South of England with the later one might throw light on this 

 question, if the stations can be identified with suflicient accuracy. No 

 minute re-measurement of a base-line would be necessary for this test. 

 If the movement is going on at all it must be far greater in a north and 

 south than in an east and west direction — i.e., it will alter the latitude 

 but not the longitude. It must therefore distort every triangle which can 

 be re-observed from two such points as St. Catherine's Down and the top 

 of Portland. 



