Chalk Strata of Ballard Head. 469 



Bottom to Chapman's Pool, on the west side of St. Adhelm's 

 Head, if prolonged, passes through the separation of the 

 chalk at Three-forked Down, between Swanwich and Stud- 

 land ; through a corresponding crack in the plastic clay at 

 Studland, and striking the Hampshire coast or Boscombe 

 Chine, north of Bourne Mouth. These two lines are the 

 parallels of nearly all the drainage channels in the adjacent 

 country : the general direction of Poole and Christchurch 

 harbours, the rivers Stour and Piddle, and the brooks that 

 enter the sea at Bourne Mouth and Boscombe Mouth, being 

 in the first direction; whilst the transverse borders of the 

 bays, &c, in the harbours, and the intersections of lines con- 

 necting the headlands between Purbeck and Southampton 

 Water take the latter, which is perpendicular to the former. 



It may be observed, that, as the Purbeck ridge is not per- 

 fectly straight, but curves in its course; so the transverse 

 lines must accordingly alter : but it is a strong confirmation 

 of the above hypothesis, that the course of the Wiltshire 

 Avon is due north and south * ; and that the Vale of Pewsey, 

 in which it rises with its tributary streams, lies east and west, 

 the river forcing its way through the barrier of chalk, having 

 cliffs of upwards of 300 ft. along its course, at the foot of 

 which it flows. Any one, who shall examine the course of 

 this river, will be convinced that its channel is altogether 

 indebted to the action of causes which produced a continu- 

 ation of rents in the chalk, through which it has now formed 

 a deep and excavated bed ; and it is scarcely to be doubted 

 that these rents were formed at the time when the Purbeck 

 portion of the chalk was upheaved from east to west.f 



The exact conformation of the surfaces of the vast planes 

 that 'compose the solid mass of the country can only arise 

 from one general impression ; and that impression was given, 

 unquestionably, by the manner in which the elevatory force 

 acted in coming to light towards the south, as it would do, 

 supposing it acted in a line from west to east, producing syn- 

 chronous phenomena in directions exactly transverse to the 

 different directions of the respective portions of the elevated 

 Purbeck range. 



Stanley Green, Poole, April 26. 1837. 



* The dip of the chalk in the north of the vale of Pewsey is 45° to the 

 south. 



■J* Between its rise and Old Sarum there are on the Avon twenty-five 

 high cliffs. The distance is not so many miles. 



m m 3 



