PART IL CHAPTER XVI. 



205 



Portland Dirt-Bed 



Fossil Forest in Isle of Portland. 



formation on which it rests. First, in regard to its junction with 

 the superincumbent lower green-sand, the beds of this last, says 

 Dr. Fitton, repose in the south-east of England, conformably 

 upon those of the subjacent weald clay. There is no indication 

 of disturbance : "To all appearance the change from the depo- 

 sition of the freshv/ater remains to that of the marine shells, 

 may have been effected simply by a tranquil submersion of the 

 land to a greater depth beneath the surface of the waters."* 



Portland dirt-bed and proofs of subsidence. — But when we 

 examine the contact of the Purbeck b'^ds, or^ inferior division of 

 the wealden, with the Portland stone, or upper member of the 

 oolite, some very singular phenomena are observed. Between 

 the two formations, the marine and the freshwater, there inter- 

 venes in Portland a layer of dark matter, called by the quarry- 

 men the " Dirt," or " Black dirt," which appears evidently to 

 have been an ancient vegetable soil. It is from twelve to eighteen 

 inches thick, is of a dark brown or black colour, and contains a 

 large proportion of earthy lignite. Through it are dispersed 

 rounded fragments of stone, from three to nine inches in diame- 

 ter, in such numbers that it almost deserves the name of gravel. 

 Many silicified trunks of coniferous trees, and the remains of 

 plants allied to the Zamia and Cycas are buried in this dirt-bed. 

 (See figure of living Zamia.) 



These plants must have become fossil on the spots where they 

 grew. The stumps of the trees stand erect for a height of from 

 one to three feet, and even in one instance to six feet, with their 

 roots attached to the soil at about the same distances from one 



Fig. 195. 



Zamia spiralis ; Southern Australia.! 



* Geol. of Hastings, p. 28. 

 s 



t See Flinder's Voyage. 



