PART II. CHAPTER XVI. 



207 



Fossil Forest in Isle of Portland. 



Fig. 197. 



freshwater calcareous 



slate, 

 dirt-bed. 



Portland stone of ma- 

 rine formation. 



Section in cliff east of Lulworth Cove. (Buckland and De la Beche.) 



beds originally horizontal. (See Fig. 197.) Traces of the dirt- 

 bed have also been observed by Dr. Buckland, about two miles 

 north of Thame, in Oxfordshire ; and by Dr. Fitton, in the cliffs 

 of the Boulonnois, on the French coast: but, as might be 

 expected, this freshwater deposit is of limited extent when com- 

 pared to most marine formations. 



From the facts above described, we may infer, first, that the 

 superior beds of the oolite, which are full of marine shells, 

 became dry land, and covered by a forest, throughout a portion 

 of the space now occupied by the south of England, the climate 

 being such as to admit the growth of the zamia and cycas. 

 2dly. This land at length sank down and was submerged with 

 its forests beneath a body of freshwater, from which sediment 

 enveloping fluviatile shells was deposited. 3dly. " The regular 

 and uniform preservation of this thin bed of black earth over a 

 distance of 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."* 



The dirt-bed has been described above in its most simple form, 

 but in some sections the appearances are more complicated. 

 The forest of the dirt-bed was not everywhere the first vegeta- 

 tion which grew in this region. Tw^j other beds of carbona- 

 ceous clay, one of them containing CycadetB in an upright posi- 

 tion, have been found below it,f which implies other oscillations 

 in the level of the same ground, and its alternate occupation by 

 land and water more than once. There must have been, first, 



* Buckland and De la Beche, Geol. Trans., Second Series, vol. iv. p. 16. 

 t Fitton, Geol. Trans., Second Series, vol. iv. p. S23. 



