Geology of the Neighbourhood of Weymouth, &;c. 15 



concretions of soft burr^ and all having their roots fixed in the dirt-bed^ which 

 occurs here also of the same thickness, and in precisely the same relative 

 place, and interspersed with the same rounded fragments of limestone which 

 it contains in the Isle of Portland : the position of these stumps, at an angle 

 nearly of 4:5^ to the horizon, affords a striking proof of the elevation which 

 the strata have undergone. We find the dirt-bed also on the top of the 

 Portland stone^ in the sections of some quarries along the Ridgeway, e. g. 

 near Upway, on the north of Weymouth, and at the western termination of 

 the Portland stone near Portisham, at the distance of twenty miles west from 

 Lulworth. Dr. Buckland has found slight traces of this dirt-bed on the upper 

 surface of a stratum of Portland stone in the quarries about two miles north 

 of Thame, in Oxfordshire : it is here covered by a few feet of clay, in which 

 he found no other animal remains than fragments of some Testudo, too 

 small to point out the genus to which they belong. The recognition of this 

 very remarkable bed in a locality so distant from Portland seems to indicate 

 that it may be found to be nearly coextensive with the Portland formation 

 throughout England ; and it well merits the attention of future observers to 

 search for it in the Vale of Aylesbury, and in the two localities of the Port- 

 land stone intermediate between Oxford and Dorsetshire, namely, at Swindon 

 and Tisbury. The probability of its occurrence at Tisbury is increased by 

 the recent recognition of the Cycadeoideae at this place by Miss Benett*. 

 We consider this dirt-bed as quite decisive in forming the barrier between 

 the Portland and Purbeck formations : its deposition must have proceeded 

 during a considerable period of time, antecedently to which the districts it 

 occupies were entirely submerged beneath the sea, and subsequently to 

 which the waters again returned to overwhelm them, first with a deposit 

 of about 1000 feet of the semi-lacustrine sediments of a great estuary (in- 

 cluding the united thickness of the Purbeck series and the Wealden sands 



* Dr. Fitton has discovered this deposit on the opposite side of the Channel in the Bou- 

 lonnois, and has thus described it in the Annals of Philosophy, December, 1826. " Some traces 

 of the lowest members of the group to which these two strata (Weald clay and Hastings sand) 

 belong, and which is remarkable from its containing throughout the remains of freshwater shells, 

 are visible on the summit of the cliffs between Gris-nez and Equinen, where a thin bed occurs of 

 somewhat bituminous clay, abounding in silicified wood, the cavities of which are coated with 

 minute crystals of quartz. This bed corresponds precisely to that which exists on the top of the 

 Isle of Portland, bearing there the name of ' Dirt,' and abounding in similar wood ; and on the 

 French coast it is associated with beds of limestone, different from the stone beneath, and contain- 

 ing shells in great numbers, apparently of the genera Cyclas and Ampullaria." Dr. Fitton has also 

 recognised thin fissile beds of Purbeck stone containing freshwater shells, e. g. Cyclas and Cypris, 

 at Whitchurch in Bucks. 



