566 Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 



Beneath this insect bed we have four feet of clays and sands, and with them the estuary 

 condition seems to give place to one of freshwater beds. 



In this freshwater portion of the lower Purbecks the beds at Ridgway-hill are almost 

 entirely calcareous, with several beds of useful stone, whereas at Durlstone bay they are 

 chiefly argillaceous, without any beds of stone until we reach the (so called) " broken bands" 

 which cover the Portland dirt-bed. Of these " broken bands" I must soon speak more 

 particularly. At Ridgway-hill the beds are not broken, but on the other hand wholly 

 undisturbed, and consist of rather soft calcareous shales splitting into thin laminae like biscuit, 

 and containing impressions of wings of insects and the scales of fish ; but as far as I know no 

 shells. There is a somewhat remarkable bed not seen in the Ridgway section, but occurring 

 in Mr Roper's quarry, a few yards off, which is very variable in its thickness, and answers to 

 a singular bed at Lulworth, to which I shall refer in speaking of the broken bands. Its place 

 in the section would be below 158. 



The Portland dirt-bed is seen at Ridgway, but is thin. Yet it contains the remains of 

 the trunks of large trees, as large as I have seen at any of the numerous points where this 

 curious subaerial bed is shewn. The trees at this locality have the appearance of being much 

 decayed externally, and I have not seen any with their bark preserved. 



The surface seems to have been covered with fresh water before it was a forest, and just 

 beneath the ancient mould is a band crowded with the scales of fish (Histionotus breviceps.) 

 Its thickness is only four inches. The fish may perhaps have died suddenly when their 

 habitation became dry. Then we have another four inch band, equally abounding in the 

 remains of a very large species of Archaeoniscus, but few tolerable specimens can be got. 

 This bed at a spot six miles to the east (Upton) is full of casts of a small Paludina. 



Our section ends with six inches of hard stone, invariably attached to the blocks of 

 Portland, and thus affording remarkable hand specimens, one side of which is freshwater 

 Purbeck limestone, with Cyprides and Cyclas, the other Portland oolite, containing Pectens or 

 other marine shells. 



A comparison of the principal divisions of the lower Purbecks at Durlstone and at 

 Ridgway will be somewhat thus : — 



Durlstone. Ridgway. 

 Insect beds, to fault 123 ft. Ditto, to bottom of section 95 ft. 



Before concluding I must not omit to describe the " broken bands" to which I have 

 several times already alluded. 



The freshwater calcareous shales which cover the Portland dirt-bed, and occupy at 

 Lulworth the places of the beds numbered in the Ridgway section from 135 to 158, but at 

 other points do not extend quite so high up, are in the eastern parts of our district invariably 

 broken into fragments, and recemented (one cannot say consolidated) by a sort of stalactitic 

 deposit. 



The cause of this appearance has not, I believe, been explained. To find in a series of 

 sedimentary rocks a thickness of some thirty feet or more in a state of ruin, while the beds 

 both above and below are undisturbed, is certainly very singular. 



