Geological Society. 473 



node moves to any coordinate plane, the longitude of tlie node of the 

 fixed plane in relation to any coordinate line, and the angular rate of 

 movement of the node of the orbit upon this fixed plane), and, when 

 once ascertained, may be regarded as fixed elements of the planet, 

 from which the position of the plane of its orbit can always be deter- 

 mined without the use of tables. 



I 



GEOLOGICAL SOCIETY. 



[Continued from p. 398.] 



November 3, 1858. — Prof. J. Phillips, President, in the Chair. 



The following communications were read : — 



1. "On some Natural Pits on the Heaths of Dorsetshire." By 

 the Rev. O. Fisher, M.A., F.G.S. 



On Affpuddle Heath and Piddletown Heath, near Dorchester, 

 the surface is pitted with numerous circular or oval hollows, like 

 inverted cones. They usually vary from about 60 to 80 yards in 

 circumference ; but one measures 130 yards, and another, called 

 " Culpepper's Dish," is 290 yards round : in the former the sloping 

 sides are 23 yards high ; in the latter 47 yards. After observing 

 that these pits could not have been formed by the washing away of 

 the underlying sand-beds, the author proceeded to show that their 

 formation seemed to be due to the subsidence of the material into 

 " sand-pipes " in the subjacent chalk, owing to the percolation of 

 rain-water containing carbonic acid, which dissolved the chalk ; and 

 Mr. Fisher referred to the explanation of this process given by 

 Mr. Prestwich in a paper formerly read before the Society. 



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 reraarlcs, that the larger pits on the Heaths 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, and must therefore have formed islands or head- 

 lands in the sea, which last filled the adjacent valleys and gave them 

 their present configuration. 



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 wliich the subsidence of the materials gave 

 rise to the peculiar shape of the pits ; and he observed that some- 

 what similar conical depressions have been noticed in process of 

 formation at the present day. 



2. "Notice of the occurrence of an Earthquake along the Nortliern 

 Edge of the Granite of the Dartmoor District, on September 28, 

 1858." By G. Wareing Ormerod, Esq., F.G.S. 



The shock was slight, and appears to have been confined to a very 

 narrow district, that may be estimated as not exceeding S miles 



