§ 1G. THE ISLE OF PORTLAND. 385 



Purbeck,* and other parts of England. The oolitic lime- 

 stone, which lies immediately upon the clay, contains 

 nodules and veins of flint and chert. The middle group 

 is full of the usual oolitic shells ; and the upper series, to 

 within twenty feet of the surface, consists of the fine archi- 

 tectural stone which is locally termed the white-led. Upon 

 this is a stratum, three or four feet thick, of limestone, full 

 of cavities left by numerous shells, of which the casts only 

 remain ; and above is an irregular layer of flint nodules, 

 in coarse oolite, which is covered by a thin bed of earthy 

 detritus. 



16. The petrified forest of Portland. — The fresh- 

 water strata which form the northern brow of the Isle of 

 Portland, belong to the lowermost beds of the Purbeck 

 group of the Wealden formation. They are in fact the 

 first sediments that were deposited by the body of fresh 

 water, which covered for ages the ancient oolitic rocks ; 

 and they indicate the commencement of the delta, which 

 ultimately spread over a great part of England and northern 

 Germany ; they are therefore, in every point of view, in 

 the highest degree interesting. The annexed section of a 

 quarry (Lign. 84) in which these strata are seen in their 

 natural position, will serve to exemplify the following 

 remarks. 



The lowermost beds in this quarry consist of the Portland 

 building stone (8) ; and upon these are layers of rubbly 

 oolite (7), covered by a thin seam of black earth, with 

 interspersed vegetable matter, in which fossil cycadeous 

 plants have been discovered. This is succeeded by lami- 

 nated freshwater limestone (o), upon which is the remark- 

 able stratum, Ccilled "Dirt-bed" (4), by the quarrymen. 

 This bed is about one foot in thickness, and consists of a 

 dark brown friable loam, containing a large proportion of 

 earthy lignite, and like the modern soil of the Island, many 



* Geo-. Exc. Isle of Wight, p. 401. 

 G C 



