552 ME. A. STRAHAN ON OVERTHRUSTS OF [Nov. 1 895,- 



over the vertical strata below it, and not depressed relatively to- 

 them, as would be the case in a fault hading to the downthrow. 

 The inference follows that the curve in the strata above the fault 

 was part of the lower limb of the monocline, and that it has been 

 brought up from that position, and placed as we now see it, partly 

 against and partly over the central and vertical portion of the 

 monocline. 



The extent of the movement along the fracture could be esti- 

 mated if we could determine the depth below sea-level of the lower 

 limb of the monocline. In filling in the structure below sea-level 

 in the section (PI. XVIII.), I have represented the vertical strata 

 as curving to horizontality at as small a depth as seems probable, 

 and the extent of the thrust as being about 400 feet. If the vertical 

 strata do not turn so soon as represented, the thrust must be greater. 

 But we seem to reach a limit in this direction, for the following 

 reason. The vertical strata and the thrust-plane diverge downwards 

 rapidly, the thrust-plane tending to curve away northward as it 

 descends. Unless the strata curve also and still more quickly, so as 

 to become parallel to and meet the thrust, we should be compelled 

 to assume that Tertiary strata come in between the two, in their 

 proper position next to the vertical Bdemnitella-zones. There 

 would thus be a lenticular mass of Tertiary rocks embedded in the 

 heart of the Chalk. Theoretically this is no more impossible than 

 that the Belemnitella-zon.es, should so occur, but it would mean that 

 the thrust-plane in some part of its course left the Chalk and tra- 

 versed the lower Tertiaries, which is not probable. That the thrust- 

 plane should in part have followed the bedding and in part cut 

 across it seems to follow from the nature of the movement along 

 the plane. It would originate as part of the hinge-like movement 

 of each bed over its neighbour, which must take place in every 

 fold. Subsequently it played a more important part in relieving a 

 strain which was tending to produce inversion, and in so doing cut 

 across the loops of the S, so as to admit of sliding on a large scale. 1 



Section 2 (PI. XVIII.) is drawn across the same line of disturbance 

 at Lulworth Cove, 14 miles farther west. The observations on 

 which it is founded were made in the east of the Cove, but the 

 section has been reversed to facilitate comparison with the others. 

 It illustrates a modification in the curves of the monocline, but 

 more especially shows how the soft strata have been squeezed out 

 of one part of the fold, where the compression has been greatest, 

 and packed away in another part, where there has been a tendency 

 to gape. 



The Portland and Lower Purbeck strata have a normal dip of 

 20° to 30°, but the Lower Wealden are inverted 20° beyond 

 verticality. The bedding of the one formation therefore stands at 

 right angles to that of the other, and would meet it, if prolonged,- 



1 In the ' Geology of the Isle of Wight,' Mem. Geol. Surr. 2nd ed. 1889, 

 p. 241, a somewhat similar thrust is described as the most probable explanation 

 of the absence of certain beds at Ashey. 



