16 Prof. BucKLAND and Mr. De la Beche on the 



and clays)j and afterwards with a series of marine deposits amounting to much 

 more than 1000 feet of greensand and chalk. 



Throughout the entire succession of all these changes there is no evidence 

 of any sudden and violent disturbance causing either elevation or depression 

 of the Portland stone, or of the subjacent oolites. The present high incli- 

 nation of all these beds is uniformly parallel to that of the beds of Purbeck 

 stone, greensand, and chalk ; and these all seem to have been raised simul- 

 taneously by the same convulsion which elevated the axis of the Weymouth 

 district, together with all the inclined strata in Purbeck and the Isle of 

 Wight. 



We have a measure of the duration of the period during which the surface 

 of the Portland stone continued in the state of dry land, covered with forest, 

 in the thickness of the " Dirt Bed," which has accumulated more than a foot 

 of black earth, loaded with the wreck of its vegetation. The regular and 

 uniform preservation of this thin bed of black earth over a distance of so 

 many miles, shows that the change from dry land to the state of a freshwater 

 lake or estuary was not accompanied by any violent denudation or rush of 

 water, since the loose black earth, together with the trees which lay prostrate 

 on its surface, must inevitably have been swept away had any such violent 

 catastrophe then taken place*. 



* Prof. Henslow in the summer of 1832 found in the top cap of the Portland stone, imme- 

 diately beneath the dirt-bed, root-shaped cavities descending from the bottom of the dirt-bed into 

 the subjacent solid stone ; this top cap should seem, therefore, to have been occupied by the 

 roots of the trees which grew in the dirt-bed, and penetrated the Portland stone whilst it was yet 

 soft and unconsolidated. He also noticed in Portland two partial and very thin seams of black 

 earth; the uppermost at the distance of five feet, and the lowermost of seven feet, below the dirt- 

 bed : these seams of black earth are important, as they mark two short intervals during which 

 vegetable matter had begun to accumulate on the surface of the soft and gradually increasing ma- 

 terials of the uppermost beds of the Portland stone, whilst they were just rising above the level of 

 the sea. The incipient bed of vegetable matter was thus twice interrupted in its progress, and 

 buried beneath an influx, first of two feet, and next of five feet, of earthy sediment, before the 

 general surface on which the true dirt-bed rests had been raised entirely above the water. 



The above cut represents an example which in the summer of 1832 occurred to Prof. 

 Henslow in the Isle of Portland, in a quarry where the surface of the burr had been laid bare, 



