78 OOLITE. 



other situations, there is a thin stratum, usually called by 

 workmen the dirt-bed, which appears, from incontestable evi- 

 dence, to have been a soil formed, like soils of the present day, 

 in the course of time, upon a surface which had previously 

 been the bottom of the sea. The dirt-bed contains exuviae of 

 tropical trees, accumulated through time, as the forest shed its 

 honours on the spot where it grew, and became itself decayed. 

 Near Weymouth there is a piece of this stratum, in which 

 stumps of trees remain rooted, mostly erect or slightly inclined, 

 and from one to three feet high ; while trunks of the same 

 forest, also silicified, lie embedded on the surface of the soil in 

 which they grew. 



Above this bed lie those which have been called the Wealden, 

 from their full development in the Weald of Sussex ; and 

 these as incontestably argue that the dry land forming the 

 dirt-bed had next afterwards become the area of brackish 

 estuaries or lakes partially connected with the sea ; for the 

 Wealden strata contain exuviae of fresh-water tribes, besides 

 those of the great saurians and chelonia. The area of this 

 estuary comprehends the whole south-east province of Eng- 

 land. A geologist thus confidently narrates the subsequent 

 events : " Much calcareous matter was first deposited [in this 

 estuary], and in it were entombed myriads of shells, appa- 

 rently analogous to those of the vivipara. Then came a thick 

 envelope of sand, sometimes interstratified with mud ; and, 

 finally, muddy matter prevailed. The solid surface beneath 

 the waters would appear to have suffered a long-continued and 

 gradual depression, which was as gradually filled, or nearly so, 

 with transported matter ; in the end, however, after a depres- 

 sion of several hundred feet, the sea again entered upon the 

 area, not suddenly or violently for the Wealden rocks pass 

 gradually into the superincumbent cretaceous series but so 

 quietly, that the mud containing the remains of terrestrial and 

 fresh-water creatures was tranquilly covered up by sands re- 

 plete with marine exuviae." 1 A subsequent depression of the 

 same area, to the depth of at least three hundred fathoms, is 

 believed to have taken place, to admit of the deposition of the 

 cretaceous beds lying above. 



From the scattered way in which remains of the larger 

 terrestrial animals occur in the Wealden, and the intermixture 

 of pebbles of the special appearance of those worn in rivers, it 



1 De la Beche's Geological Researches, p. 344. 



