Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 



557 



which appear at Brixton bay are some beds of white sandstone, the apparent equivalents 

 of which are not half way down the Swanage section, while the impression conveyed to 

 the mind is, that the beds exposed in the former locality are nearly as thick as the whole 

 Swanage series. At Worbarrow bay the thickness is much less than it is at Swanage ; 

 and again on the west side of Worbarrow less than on the east. The diminution is very 

 striking as we pass on to Lulworth Cove. At Ridgway-hill the fault cuts off the upper 

 portion, but the part that is left (about 350 feet) does not militate against the truth of 

 this assertion*. 



It must be owned, however, that the unconformable superposition of the upper 

 green sand and gait, which form the superior limit of the Hastings sand, somewhat lessens 

 the strength of the argument drawn from the change in thickness. The disappearance 

 of the marine bands in the upper part of the series which occur in the Island, and at 

 Punfield in Swanage bay, is in accordance with either supposition, that we are receding 

 from the ancient sea, or that the upper beds have been cut off as we proceed westward -f\ 



I am not able to give any information respecting the part of the series in which the 

 Iguanodon's bones have been found at any of the localities I have mentioned except 

 Ridgway. There they occurred rather abundantly in a stiff grey clay near the base of 

 the series, just above the passage beds which connect these sands and clays with the 

 upper Purbecks. 



With these few remarks upon the Hastings sands as seen in Dorsetshire, I will now pro- 

 ceed to describe the Purbeck beds. 



The localities to which I shall refer are Durlstone bay, close to the town of Swanage ; 

 Worbarrow forming the Eastern headland of the bay of that name, ten miles westward ; Mupes 

 bay and Bacon hole, at the west of the same bay, a mile and a half across; Lulworth Cove, 

 a mile further, with Ridgway and Upwey ten miles still further to the west. Hence in the 

 whole district we include a range of about three and twenty miles. As might be expected, 

 the lithological character of the deposit varies considerably within these limits. The grandest 

 development is undoubtedly that of Durlstone bay. At Worbarrow the development still 

 resembles that at Durlstone bay, but in crossing to Bacon hole the deterioration of the series 

 is very perceptible, and the development on the whole is not greater than that at Ridgway — 

 the other extremity of our district. 



Professor E. Forbes has divided <the Purbeck series into upper, middle, and lower, and 

 has found that these several divisions are " each marked by a peculiar assemblage of organic 

 remains j." Following his division, we shall find that the upper and middle divisions are 

 most largely developed at Durlstone bay, while the lower (at least towards the bottom of it) 

 is most developed at Ridgway, and to a casual observer it is striking that all the stone-quar- 

 ries at Swanage are in the middle Purbecks (except the marble in the upper), while at Ridgway 



* Unios occur both at Swanage and Ridgway in some hard, 

 conglomerate-looking bands. 



•f At Punfield there is undoubtedly a fault separating the 

 gait from the Hastings sand. I saw it distinctly in a small 



sand-pit on the 30th of June, 1853. The gait following the 

 chalk and green sand is vertical, while the Hastings sands dip 

 at a high angle towards the north. 



X British Association Reports, I860, p. 79. 



