SPECIAL GEOLOGY. 



397 



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Portland stone for building, on which rest the lowest fresh- 

 water beds, and a mass of bituminous earth, called the 

 "dirt-bed," which is an ancient vegetable soil, containing 

 numerous trunks of fossil trees, standing erect at a height 

 of from one to three feet, with their summits jagged, as 

 if they had been broken off by a hurricane. (Fig. 276.) 



They are placed at the same distance ^ S ^ 



from each other as trees usually occupy 1 | 



in a forest, and have their roots at- ^ 3 

 tached; showing that they now occupy 

 the spot on which they lived and 

 flourished. 



The same bed of earth contains many 

 remains of Cycadea, allied to the recent 

 Cycas and Zamia. Prom their rounded 

 shape, they are called birds' nests by the 

 workmen. (Fig. 279.) 



The "dirt-bed "can also betraced on the 

 mainland, being seen in the same relative 

 position in the cliffs of Lulworth Cove, 

 on the coast of Dorsetshire, where, how- 

 ever, the strata having been inclined to 

 an angle of 45, the dirt-bed, with its 

 fossil trees, is inclined with them ; pre- 

 senting the most convincing evidence of 

 the disturbance of strata, and a change 

 in the position of beds originally hori- 

 zontal, since it is obvious that the trees 

 must have grown erect, and subsequently 

 have been inclined with the bed in which 

 they grew. Traces of the dirt-bed have 

 been observed in Oxfordshire, and like- 

 wise in the cliffs of the Boulonnois, on 

 the French coast. 



It has been ascertained that the forest 

 of the dirt-bed was not the earliest ve- 

 getation of this locality. Two beds of 

 carbonaceous clay, one of them contain- 

 ing Cycadece in an erect position, have 

 been discovered below it, and one above 

 it; which fact, with others furnished by the organic 



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