DISTURBANCES AND FAULTS. 241 



connection exists between them. The Palseozoic Rocks had 

 already been folded when the Secondary Period commenced, while 

 the folds with which we are concerned were produced in a late 

 Tertiary age. It is, however, possible that the direction of the 

 later folds was influenced by that of the earlier set, for the old 

 rocks may have yielded more readily along the former lines of 

 flexure, than along new lines crossing these obliquely. 



We have already noticed the sudden downward plunge of the 

 beds on the north side of all the anticlines. This form of fold 

 seems to be the first stage in the formation of a thrust-plane or 

 slide-fault. For though in the Isle of Wight the movement has 

 not usually gone further than to produce verticality of the beds, yet 

 on following the fold across to Dorsetshire, that is nearer the area 

 of greatest movement, we meet an instance of an actual thrust- 

 plane in the Chalk. This dislocation was first noticed by Mr. 

 Webster in 1811, and described and figured by him in Englefield's 

 Isle of Wight (pp. 164-168, PI. 26 and 27). The cliff of Hand- 

 fast Point is formed in the southern part of vertical beds, and 

 iu the northern of nearly horizontal beds of chalk. The hori- 

 zontal strata, as they approach the vertical series, turn upwards in 

 a great curve, forming nearly the quarter of a circle. A fracture 

 has taken place, exactly following one of the curved bedding- 

 planes, and the curved and gently inclined beds have been pushed 

 bodily over the edges of the vertical beds, so as now to rest 

 upon them with an nppearance of an extreme unconformity. 

 The bedding of the vertical strata seems at a distance to be 

 regular, with the lines of flints in their usual condition. But on a 

 closer view, the chalk is found to be entirely reconstructed. The 

 flints are not only broken to fragments, but the fragments are more 

 or less separated from one another, while the entire mass of chalk 

 is traversed by veins of calc-spar, and by planes of slickenside 

 filled in with secondary flint. The chalk, moreover, has been 

 hardened to the consistency of limestone. 



No trace of a similar thrust-plane is found at either end of the 

 Isle of Wight, but at Ashey the close proximity of fossiliferous 

 strata, probably representing the middle part of the Bracklesham 

 Series, to the basement bed of the London Clay, shows that a strike 

 fault of a peculiar character must there be present. The bedding 

 on each side of the presumed line of fault is perfectly vertical, 

 and to account for the absence of about 400 feet of clays and 

 sands the simplest explanation seems to be that adopted in the new 

 edition of Sheet 47 of the Horizontal Sections now in preparation 

 — that at Ashey a thrust-fault occurs, and that its form and 

 effect on the beds correspond closely with what we know is found 

 on the mainland. Even at a considerable distance from the belt 

 of highly inclined rocks, in the Tertiary Beds of the Isle of Wight, 

 small thrust-planes are occasionally found in the harder strata 

 (see Fig. 83). 



E 56786. 



