46 THE PURBECK BEDS OF THE VALE OF WARDOUR. [Feb. 1 894. 



interrupted by cross-rolls, and apparently also by small faults, that 

 the relative position of a section is no guide to its real geological 

 position. The consequence is that, unless some particular bed can 

 be identified with one seen elsewhere, it is very difficult to refer an 

 isolated exposure to its proper place in the series. There are a few 

 such sections about the position of which we are still uncertain, 

 but we think the beds seen in them will prove to be only the 

 equivalents of some of the strata described in this paper. 



We are indebted to Prof. T. Eupert Jones, F.E.S., and Mr. C. 

 Davies Sherborn for identifying the Cyprids which we collected, 

 and to Messrs. G. Sharman and E. T. Newton, F.B.S., for naming 

 some of the mollusca. 



II. General Structure of the District. 



The main features of the geological structure of the Vale of 

 Wardour are well known. It will suffice to say that the Purbeck 

 Beds come in at the eastern end of the Vale, because an easterly 

 dip was imparted to the Jurassic system before the superposition of 

 the Cretaceous strata upon it. The uncovering of the Purbeck Beds 

 at this locality is due to the denudation which has taken place 

 along the axis of a post-Cretaceous anticlinal flexure. 



The axis of this anticlinal does not, however, coincide with the 

 lowest or central part of the valley, but runs nearly due east-and- 

 west through Lady Down and Teffont Evias, its northern limb 

 being much shorter and more steeply inclined than its southern 

 limb. Moreover, the pre-Cretaceous dip of the Jurassic strata was 

 not due east, but nearly south-east ; neither was this south-easterly 

 dip a steady and continuous one, but interrupted by cross-rolls and 

 faults, so that the present disposition of the outcrops results from 

 the interference of three distinct sets of disturbances. The conse- 

 quence of this interference is to produce a number of divergent 

 dips, and to make the mapping of the subdivisions of the Purbeck 

 series rather complicated. 



Although the beds are horizontal on the top of Lady Down, 

 they slope thence rapidly south-eastward, and the base-line of the 

 Middle Purbeck group falls through 200 feet in less than two miles, 

 owing to the combined effect of the original dip and the inclination 

 of the southern limb of the post-Cretaceous anticlinal. 



The axes of the pre-Cretaceous flexures strike from north-west to 

 south-east, and going from west to east the following flexures may 

 be noticed : — 



(1) The anticlinal of the Chilmark Valley, which brings up the 

 Portland Beds near Chilmark, but seems to die away southward, or 

 is masked by the steep south-easterly inclination of the axis. 



(2) A synclinal bringing in what seems to be the base of the 

 Upper Purbeck group in a small cutting on the railway near 

 Daslett Farm. 



