406 Proceedings of Philosophical Societies. [Junb, 



valleys of Pewsey near Devizes, and of the Wily and the Nadder 

 above Salisbury, have also, to a certain degree, been affected 

 by a force acting from beneath, and elevating the strata at a 

 period antecedent to their being submitted to denudation ; and 

 concludes, that not only these enclosed valleys similar to that 

 of Kingsclere, but many open valleys also (though in all cases 

 modified by subsequent denudation), had a prior origin arising 

 from the fracture and elevation of their component strata : this 

 must have happened in the case of the Weald of Kent and 

 Sussex, enclosed as it is with an escarpment of chalk dipping 

 every where outwards in opposite directions, and sometimes 

 very rapidly, along the North and South Downs. 



The author proceeds to illustrate, by the position of the strata 

 of plastic clay in this same district, the important Geological 

 question whether the chalk was disposed in its present form 

 of troughs or basins, before or after the deposition of the 

 tertiary formations now enclosed in them, and to show that 

 the present inchnation of the strata along the S frontier of the 

 basins of London and Hants took place since the deposition 

 of the plastic, and probably also of the London clays ; and 

 that these two basins were once connected together across the 

 now intermediate chalky strata of the downs of Hants, Wilts, 

 and Dorset; since it appears that the plastic clay formation is 

 so far from being limited to the lower levels of the present 

 basins, that large residuary fragments of it still occur on the 

 summits of the most elevated portions of chalk in these coun- 

 ties, e. g. on the summit of Inkpen near Newbury, and on that 

 of Blackdown near Abbotsbury, as well as on the top of Chid- 

 bury and Beacon hills in the highest part of Salisbury plain. 

 The strata that covered the intermediate spaces have probably 

 been removed by diluvial denudation, and the destructible 

 nature of their component materials would render them pecu- 

 liarly liable to be swept away by the transit of violent currents 

 of water. The wreck of the harder portions of the sandy 

 strata thus destroyed, forms the sandstone blocks called grey 

 weathers that lie loosely scattered on the naked surface of the 

 chalk in all these counties, and of which Stonehenge is con- 

 structed. In lower levels, within the existing basins, these 

 same strata have been less destroyed, in consequence of the 

 greater protection from the ravages ofdiluvial denudation which 

 their low position has afforded them. 



The author concludes by referring to the occurrence of 

 similar tertiary strata as well as of chalk and green sand on 

 the summit of the Savoy Alps, nearly 10,000 feet above the 

 level of the sea, where they seem to bear the same relation to 

 the tertiary strata of the valleys of Italy, France, and Ger- 

 many, that our trifling elevations of Inkpen, Blackdown, &c. 

 bear to the basins of London and Hants, and concludes that 



