S6 Prof. BucKLAND and Mr. De la Beche on the 



of chalk, greensand, and Portland stone, to a position lower than the strata 

 from whose extremities they have been snapped otf *. 



1. Great Ridgeway Fault. 



The most important of these, the great Ridgeway fault, extends, without 

 interruption, nearly fifteen miles, from the eastern extremity of our district at 

 Moignes Down, to the sea near Abbotsbury, passing along the great escarp- 

 ment of the chalk at various elevations, from the top to the bottom of it ; the 

 Osmington fault we could trace only about three miles, from South Holworth 

 near White Nore, to Ham Cliff on the north-east of Weymouth. 



The first or great Ridgeway fault is one of the most curious and important 

 we have ever seen, in consequence of the variety of instructive sections 

 afforded along its course ; these sections we shall describe in the order of 

 their occurrence, beginning at the eastern extremity f. The fault emerges 

 from the chalk formation at Moignes Down Farm, on the north side of the 

 circus of Moignes Down, and brings the truncated lower ends of strata of 

 Portland stone into contact with the truncated upper ends of strata of chalk, 

 both dipping to the north |. Here a valley of denudation runs exactly along 

 the line of fault, having its north side composed of chalk, and its south side 

 entirely of Portland stone. The strata have been raised on both sides of this 

 fault, but raised unequally ; whence it results, that on the north side the 

 chalk rises towards the fault, whilst on the south side the Portland stone 

 dips towards it, as if plunging beneath the chalk ; whereas the Portland 

 stone has been elevated from its original position, relatively, though not 

 absolutely, much higher than the chalk : yet, notwithstanding this fracture 

 of the strata and dissimilarity of substances on the two sides of the valley, 

 the effect of denudation has been so equable, and the removal of the fractured 

 materials so total, that no other features are presented by the surface than 

 those of an ordinary valley of denudation on horizontal chalk. 



Our next section § is taken at Poxwell Hill, less than one mile west of 

 Moignes Down Farm. Here the circumstances of the fault on the north of 

 Poxwell Circus are similar to those at Moignes Down Farm, excepting that 

 at Poxwell there has been no denudation on the line of fault. Two miles 

 west of Poxwell in the valley of Sutton Pointz||, the exact line of the fault is 

 scarcely to be recognised, from the circumstance of the strata on both sides 

 of it being perpendicular. At fig. 5. in Sutton Valley, one mile west of 



* See Plate IT. fig. 2, 3. f See Plate II. fig. 1, 2, 3, 4, 5, 6, 7, 12. 



+ See Plate II. fig. 2. § Plate II. fig. 3. H Plate II. fig. 4. 



