Dr. FiTTON on the Strata below the Chalk. 269 



Wood. Silicified specimens of coniferous wood, in fragments, the cavities of which 

 are lined with minute crystals of quartz, are found in the debris of the great 

 quarry at Swindon : they are supposed by the quarrymen to have been found 

 within the solid stone; but there can be scarcely a doubt that they have fallen 

 from the beds above, the place of which corresponds to that of the group which 

 affords the silicified trunks in Portland. 



[^Portland Sand.'} 

 Ammonites biplex. Swindon. 

 Osirea (deeply concave). Swindon. 

 Pecten. Swindon. 



Perna quadrata. Swindon : with Mya ? 

 Pullastra. Swindon. 

 Trigonia davellata. Swindon : in bluish sandy clay; with pebbles of black flint or 



Lydian stone. 

 T gibbosa. Casts in dark flint, with Pecten or Lucina ? 



Bone. Portions of a long bone of a Saurian. 



\_Kimmeridge Clay.'] 

 Ammonites. Near Steeple Ashton : in slaty lignite, like the Kimmeridge "Coal". 



Berkshire, Oxfordshire, Buckinghamshire, Bedfordshire. 



(143.) The tract between Calne and Bedfordshire derives its exterior cha- 

 racter in a great measure from the very slight inclination of the strata^ which 

 in their progress towards the north-west from the anticlinal line of the Vale 

 of Wardour, are comparatively undisturbed : no conspicuous faults^ at leasts 

 have hitherto been detected between South Wiltshire and the northern coast 

 of Norfolk, though it is probable that this region is not more exempt from 

 slight disturbances, than the greater part even of the more uniform tracts in 

 England is found to be, when examined with sufficient attention. One effect 

 of this very gradual rise of the strata is, that some of the beds, shooting 

 out successively to great distances, have been the subject of extensive de- 

 nudation ; so that outliers occur in several places at the distance of some 

 miles from the general line of the escarpments : and on the confines of Berks 

 and Oxfordshire a large tract, entirely denuded of chalk, appears, along the 

 course of the Thames between Abingdon and Reading, breaking in upon 

 the almost uniform and linear course which the outcrop of that formation 

 would otherwise have followed from Calne to the fens of Cambridgeshire. 

 It is not improbable that this wide denudation, and the course of the Thames 



