Geology of the Neighbourhood of Weymouth, Sgc. 



13 



lowest of them form the surface of the island^ and cover the true Portland 

 stone with a deposit of freshwater formation. This formation presents beds 

 of chert containing freshwater shells, near the Ridgeway fault, on the west 

 side of the road from Weymouth to Dorchester; and also beds of chert con- 

 taining chalcedonic casts of minute freshwater shells, at the water's edge, on 

 the east side of Lulworth Cove. 



Section of the Dirt-bed in the Isle of Portland. 



Calcareous slate. 



Dirt-bed. 



Portland stone. 



^ Freshwater formation. 



Ancient forest. 



Marine formation. 



We consider a small stratum, called by the workmen "Dirt-bed," to be by 

 far the most interesting and remarkable deposit in this district. It seems to be 

 made up of black loam, mixed with the exuviae of tropical plants, accumulated 

 on the spot on which they grew, and preserved during a series of years, in 

 vi^hich the surface of the Portland stone had for a time become dry land, and 

 accumulated a soil of about a foot in thickness, composed of an admixture of 

 earth and black vegetable matter, interspersed with slightly rounded frag- 

 ments of stone, which Mr. Webster ascertained to be from the lower part of 

 the Portland series. These fragments are found to be almost coextensive 

 with the dirt-bed, and the fact that we have yet found with them no admix- 

 ture of pebbles derived from the subjacent oolites, or from any other more 

 ancient rocks, shows that no violent rush of water from any distant region 

 took place during the period in which these pebbles of Portland stone were 

 under the process of becoming slightly rounded. 



This dirt-bed, as Mr. Webster has stated, forms the matrix of the silicified 

 trunks of very large coniferous trees, which are so abundant in the Isle of Port- 

 land, and are found there coextensive with the upper surface of the Portland 

 stone. Wherever the dirt-bed is laid open to extract the subjacent building- 

 stone, it is found to contain these silicified trees laid prostrate, partly sunk 

 into the black earth, and partly covered by the superjacent calcareo-siliceous 

 slate : from this slate the silex, to which the trees are now converted, must 



