Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 



567 



At Portland, Ridgway, Osmington, Upton, and Ringstead, this broken condition of the 

 beds in question is not found, but at Lulworth Fig. (2).* 



it is very remarkable, and also at Round down 

 and Durlstone bay. 



The result of an examination I made of 

 these beds at Lulworth was as follows : — 



The stumps of the trees sometimes remain 

 in a silicified state, but more frequently they 

 have disappeared. The spot where each stood 

 is surrounded by a dome-shaped swelling of 

 the bed of porous limestone which covers the 

 dirt-bed. These domes have generally a pit in the centre where the trunk protruded, but 

 some have not. Such perhaps may have covered Zamias. Other swellings of the bed are 

 long and trough-shaped, having accumulated about fallen trees. The surface of this bed of 

 stone is very irregular, and it has much the character of the deposit from a petrifying spring 

 on moss. No carbonaceous matter is present, except in the ancient soil beneath. Above 

 this uneven bed all the strata are broken up until we reach the cardium-bed, No. 134 of the 

 Ridgway section, which is undisturbed. The disturbance appears to diminish in intensity as 

 we approach this limit. 



Among the broken strata occasional horizontal bands of chert occur, apparently due to a 

 subsequent solidification of some sandy beds amongst them. 



It is clear, therefore, that the fracture of these beds took place before the cardium-bed 

 was deposited, and that sufficient time elapsed for them to become solidified before their 

 fracture. I would suggest the following explanation of the phenomena. 



The ground, continuing to sink after the subaerial condition in which the forest grew, 

 became swampy, and mosses grew ovsr it, which crept, up the stems of the trees, and formed 

 hollow cones around them, and were encrusted with carbonate of lime. The trees then 

 died, and, their branches falling, formed a layer of wood and twigs over the ground. The 

 surface, gradually saturated with wet, became a lake or river-bed, and calcareous mud and 

 sand were spread out in layers over the vegetable stratum. The latter not becoming 

 bituminized, but continuing to decay slowly, gave way, but not equally, the stems of the trees 

 perhaps standing longest. ' Hence, as the upper beds fell in upon the vacuity, they did not 

 settle regularly, but were broken up. The upper beds are more disturbed than the lower 

 ones, which would agree with this hypothesis. A stratum of clay immediately beneath the 

 cardium-bed dips down among the inequalities of the broken bands, and affords a level surface 

 for the cardium-bed to repose upon. 



(a) Portland "dirt" bed. 



(6) Porous bed (soft cap) ; shewing the hollows left by 



the trunks of trees. 

 (c) Broken bands, with a lenticular layer of chert in sand. 



Vol. IX. Part IV. 



(d) Clay on which reposes 

 (c) Cardium bed undisturbed. 

 (/) Calcareous shales. 



73 



