GEOLOGY. iZo 



bon&te of lime, which after precipitation, has left a calcareous 

 deposit, of a purity in proportion to the absence or presence 

 of vegetable matter. 



During the deposition of No. 1, the lake was probably of con- 

 siderable depth, as the tufa appears unmixed with any other body. 

 No. 2, is darkened with vegetable matter; and No. 3, appears 

 to have been formed when the lake was extremely shallow, and 

 favorable to the growth of aquatic plants, around decayed 

 portions of which the lime accumulated, occasioning the singular 

 cavities with which it is interspersed. Upheaval and accumula- 

 tion would in process of time raise the bed, and compel 

 the stream to find out for itself a new channel, leaving the area 

 a marshy swamp, favourable to the habits of Helix sericea, 

 Succinea oblonga, &c., and to the formation of the loam which 

 surmounts this extraordinary bed. 



To us, who live in these days of enquiry and investigation of 

 natural phaenomena, is given the task of accounting for the asso- 

 ciation of fresh water and terrestrial shells in a valley bounded 

 by two ranges of hills, and several feet above the level of the 

 sea, (from which it is four miles distant ;) and I trust the subject 

 may attract the attention of scientific men, whose powers of 

 deduction may satisfactorily solve this interesting problem. 

 Mr. Bristow, an eminent geologist, who has worked up the 

 Purbeck and Wealden fresh water and fluvio-marine strata of 

 Dorsetshire, and added considerably to our acquaintance with 

 the geology of the district, has examined the Blashenwell 

 bed, and agrees with me as to its tiifaceous character, but he con- 

 siders the introduction of the marine shells to be caused by sea 

 fowl, in which opinion the Rev. J. H. Austen shares ; but, as 

 the lake was comparatively small, and the periwinkles and 

 limpets are abundantly distributed, I think upon further ex- 

 amination, they will be induced to alter their opinion. 



JOHN C. MANSEL. 



