188 PEOCEEDINGS OF IHE GEOLOGICAL SOCIETY. [NoV. 3, 



the subjacent chalk, owing to the percolation of rain-water con- 

 taining carbonic acid, which dissolved the chalk ; and Mr. Fisher 

 referred to the explanation of this process given by Mx. Prestwich 

 in a paper formerly read before the Society*. 



It is evident, that wherever a sand-pipe has been formed there a 

 pit must have appeared on the surface, though the bottom of the 

 pit may have been many feet above the chalk- surface ; as we see a 

 conical depression formed in the surface of the sand in the upper bulb 

 of an hour-glass as soon as it begins to run. The Heaths in ques- 

 tion would, therefore, illustrate the normal condition of every sur- 

 face covering sand-pipes ; and, if no pits occur, the surface must 

 have subsequently been levelled by some agency or another. As 

 this process could have gone on only during a subaerial condition of 

 the surface, and must have occupied a very long time, the author 

 remarks, that the larger pits on the Heath referred to must have 

 been formed by the sinking of the Eocene beds into enormous 

 " sand-pipes" during an extended geological period, and that the 

 area they occupy was dry land during all that time, and has been so 

 ever since, or the pits would have been obliterated ; and must there- 

 fore have formed islands or headlands in the sea, which last filled 

 the adjacent valleys and gave them their present configuration. The 

 denuded chalk-valley separating the two Heaths is in most parts 

 destitute of gravel, and contains numerous pipes full of Eocene ma- 

 terials, proving that there was a period during which large pipes 

 were formed anteriorly to the denudation ; while the nearness of 

 the pipes to each other shows that the great number of the pits is no 

 argument against the cause assigned to them. 



As the formation of these pits was subsequent to the outspread of 

 the superficial gravel of these Heaths, and previous to the last de- 

 pression and elevation of the land, their date would be perhaps near 

 that of the great mammalian fauna. The author also explained his 

 views of the method by which the subsidence of the materials gave 

 rise to the peculiar shape of the pits (for, however deep the original 

 depression, and of whatever shape, the sides would always eventually 

 assume the angle at which the materials would stand) ; and he ob- 

 served that somewhat similar depressions have been noticed in the 

 neighbourhood in process of formation at the present day. 



2. Notice of the Occurrence of an Eauthquake along the Noktheen 

 Edge of the Grajstite of the Dartmoor District on the 28th of 

 September, 1858. By G. Wareikg Ormerod, Esq., M. A., E.G.S. 



On the evening of Tuesday, the 28th Sept. last, a slight shock of an 

 earthquake was felt in the district adjoining the northerly edge of 

 Dartmoor ; and it appears to have been almost entirely confined to 



* Quart. Journ. Geol. Soc. vol. x. p. 241. 



