322 Dr. FiTTON on the Strata below the Chalk. 



P'arnham and Petersfield and around the west end of the VVealden denu- 

 dation ; the Lower green-sand, especially, is nowhere better disclosed, except 

 in the Isle of Wight. It is not improbable, that the part of these groups 

 which once intervened between the Isle of Wight and the Sussex coast, and 

 is now concealed by the sea, was likewise fully expanded ; but at Folkstone 

 the Upper green-sand has already become much thinner. 



It remains yet to be ascertained whether in their prolongation northward 

 in England, and towards the east on the continent of Europe, these groups 

 again assume their more bulky and varied form, as our Upper green-sand 

 is observed to do after reduction, even within very short distances ; and 

 whether, if they do so expand, the relative proportions of the component 

 members be the same in remote countries ; an inquiry which is the more in- 

 teresting in the case of the green-sand formation, from its very wide diffu- 

 sion, and the great space which it occupies in different parts of Europe. 



Details, — The following are some of the local variations in the strata composing the groups 

 above referred to : — 



Chalk. — It may be remarked that the chalk rises much more rapidly towards its outcrop, 

 between Wiltshire and Hertfordshire, thnn in the more northern counties of the South East of 

 England. In departing from that central tract, both towards Devonshire on the south, and 

 northwards, the rise is comparatively small; the beds shooting out very gradually, and occupying 

 very wide spaces of uniform, flat country. The disturbance, by which the ridge of the Wealds 

 and the parallel range of elevations through the Isle of Wight and Dorsetshire were produced, 

 wasprobably the principal cause of this variation : and the mean course of the Thames itself, 

 from the point where it cuts through the chalk, may possibly be connected with that derange- 

 ment. But in the chalk itself, on the south-west of London, much local variation is found 

 in the angle and direction of the dip ; and from the facts mentioned by Mr. De la Beche, respect- 

 ing the faults in Devonshire, and what I have seen of the minor irregularities on the shore 

 near Hastings, it is probable that if the whole surface in this part of England were examined 

 with equal attention, it would prove to be pervaded by cracks and slight dislocations. 



The Upper green-sand is especially remarkable for great variation in its thickness and compo- 

 sition ; and it is everywhere so intimately connected with the lowest beds of the chalk, that 

 if we had more accurate measurements of our strata, the total thickness of that formation, in- 

 cluding the Upper green-sand — might, perhaps, be found to be nearly uniform. The latter passes 

 most commonly by insensible gradation into the soft dark-coloured marly chalk. In many cases, it 

 forms a very thin and scarcely distinguishable group, — as near Folkstone, where it is not more 

 than 20 feet thick ; in others, the chert which it contains becomes conspicuous, and it affords also 

 beds of firestone, which together giving it firmness and durability, cause it to project as a step 

 or prominence beyond the foot of the chalk range. 



On the west of Portland, the green-sands thin out so rapidly, that at Blackdown a series about 

 200 feet thick is their only representative ; and it is difficult to decide whether this be a con- 

 densed equivalent, of the whole group between the chalk and the lowest green-sand, — or, (which 

 seems to be more probable,) only a continuation of the lower part of the Upper green-sand, 

 from the Vale of Wardour. In following the range of these formations towards the north-east, 



