390 



FOSSIL FORESTS IN ISLE OF PORTLAND 



[Oh. XX 



containing Cyclas, Valvata, and Zimncea, of the same species as those 

 of the uppermost part of the Lower Purbeck, or above the dirt-bed. 



The freshwater limestone in its 

 turn rests upon the top beds 

 of the Portland stone, which, 

 although it contains purely ma- 

 rine remains, often consists of a 

 rock quite homogeneous in min- 

 eral character with the Lowest 

 Purbeck limestone.* 



The most remarkable of all 

 the varied succession of beds 

 enumerated in the above list, is 

 that called by the quarrymen 

 "the dirt," or "black dirt," 

 which was evidently an ancient vegetable soil. It is from 12 to 18 

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

 proportion of earthy lignite. . Through it are dispersed rounded frag- 

 ments of stone, from 3 to 9 inches in diameter, in such numbers that 

 it almost deserves the name of gravel. Many silicified trunks of 

 coniferous trees, and the remains of plants allied to Zamia and Cycas, 

 are buried in this dirt-bed (see figure of fossil species, fig. 376, and 

 of living Zamia, fig. 377). 



Fig. 37T. 



Cycadeoidea {MantelUa) megalophylla, 

 Bucldand. 



Zamia spiralis. Southern Australia. 



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 another as 

 the trees in a modern forest.f The carbonaceous matter is most 

 abundant immediately around the stumps, and round the remains of 

 fossil Cycadeai.\ 



* Weston, Geol. Quart. Journ., vol. viii. p. 117. 



f Mr. Webster first noticed the erect position of the trees, and described the Dirt-bed. 



X Fitton, Geol. Trans., Second Series, vol. iv. pp. 220, 221. 



