§ 18. 



DIRT-BED OF PURBECK. 



389 



belong to a distinct genus.* There are two species ; one 

 is short, and of a spheroidal form (M. nidiformw,Lign. 85); 

 the other is longer, and subcylindrical (M. cylindrical). 

 The trees and plants are completely silicified ; and their 

 internal structure is beautifully preserved in many examples ; 

 the wood, microscopically examined, displays the organi- 

 zation of the Araucaria. A cone has been found in the 

 Dirt-bed, which Dr. Brown considers to be nearly related 

 to the fruit of the Norfolk Island pine {Araucaria excelsa). 

 The Portland and Isle of Wight fossil trees, appear to 

 belong to the same species of coni ferae. 



Lign. 86. — Section of the cliff on the east of Lulworth Cove. 



1, Purbeck calcareous slate; 2, Dirt-bed, with trunks of trees ; 3, Oolitic limestone 



of Portland. 



18. Dirt-bed of Purbeck. — 

 through the northern part of the 

 appears on the coast of Purbeck 

 peculiar interest. In the highly 

 cliff, about a furlong to the east of 

 siderable number of petrified tr 

 similar conditions with those of 

 The lowermost strata {Lign. 86, 3), 



The Dirt-bed extends 

 Isle of Portland, and 

 under circumstances of 

 inclined strata of the 

 Lulworth Cove, a con- 

 ees is exposed, under 

 the Portland quarries, 

 are the Portland oolitic 



* Named by M. Adolphe Brongniart, Mantellia. 



t Specimens of the former species are called " Crows nests " by the 

 quarrymen ; who believe them to be birds' nests, originally built by 

 crows in the pine-trees, and which have since become petrified. 



