122 GEOLOGY. 



The perfect condition of the shells at Blashenwell, evidences 

 the peaceful and gradual deposition of the bed, such as is likely 

 to occur under the undisturbed waters of a pond or lake, and is 

 distinct from the coarse, heterogeneous beds of marl and chalk, 

 which occur on the high lands of the county, leading the casual 

 observer to suppose that they are detached portions of the tertiary 

 period, but when possible to obtain a section of the part, it is 

 found to be merely a large cavity filled with an aqueous deposit, 

 which some violent disturbance, such as the upheaval of 

 land would occasion. Both are subsequent to the tertiary period, 

 both are the result of mechanical, and not chemical action ; but 

 they differ in the conditions under which they were deposited — 

 the former dui'ing a period of tranquillity, the latter of dis- 

 turbance. 



The configuration of the surface over which the tufa extends, 

 was favorable to the formation of a lake, where fresh water 

 mollusca may have lived and died, and on whose margin ter- 

 restrial moUusca may have subsisted, which after becoming 

 associated with their aquatic brethren in this calcareous cemetery, 

 by means of floods and other causes, would contribute towards 

 the general debris of animal life. 



That the district under review has been upheaved, since the 

 formation of the bed, is probable, from the occurrence of a bed 

 of cockles, periwinkles, and oysters, (C. e. L. 1., and O. e.,) 

 in the church yard, at Arne, about thirty feet above the level 

 of the sea. The shells there lie embedded in the usual mud or silt 

 of Poole harbour, and do not approach the surface within several 

 feet, owing probably to a change of the tide or eddy wiiich 

 had collected them there. Previous to this upheaval, the 

 surrounding country might have been subject to transitory in- 

 roads of the sea, by which means the marine shells would gain 

 access into the lake, after forcing their way through the gorge 

 at Corfe Castle, fragments of flint from the chalk range, 

 would be introduced by the same agency, and the bones, and 

 portions of the Purbeck beds, by the drainage of the surrounding 

 high grounds. 



We must now direct our attention to the probable origin of 

 the bed, which, being tufaceous, leads to the supposition that 

 the water issuing from the spring, was highly charged with car- 



