THE CRUST OF THE EARTH. 



worn, and resembled that of posts and timbers of groins or piers 

 within reach of the tides, and subjected to the alternate influence 

 of the water and atmosphere ; there are but seldom any vestiges 

 of the bark. 



" The fossil plants related to the recent Cycas andZamia,* occur 

 in the intervals between th$ pine-trees; and the dirt-bed is so little 

 consolidated, that I dug up with a spade, as from a parterre, 

 several specimens that were standing on the very spot where they 

 originally grew, having, like the columns of the temple of Poz- 

 zuoli, preserved their original erect position amidst all the revo- 

 lutions which have subsequently swept over the surface of the 

 earth, and buried them beneath the accumulated detritus of 

 innumerable ages. These fossil plants, though related to the 

 recent Cycadese, belong to a distinct genus, f There are two 

 species one is short, and of a spheroidal form (M. nidiformis) ; 

 the other is longer, and subcylindrical (M. cylindrica). t 



" The trees and plants are completely silicified, and their internal 

 structure is beautifully preserved in many examples ; the wood, 

 microscopically examined, displays the organisation of the Arau- 

 caria. 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 Conifer a)." 



Fig. 86. Section of the Portland dirt-bed. 



170. The presence of plants analogous to the modern Cycas 

 and Zamia shows that the climate of England, at the time when 

 the vegetation of this stratum flourished, must have been 



* These plants are so common in conservatories that their general 

 appearance must be familiar to the reader. In the Botanic Gardens at 

 Kew there are magnificent specimens of Cycas and Zamia, and of other 

 plants of hot climates, of which related forms occur in the Wealden. 



f Named by M. Adolphe Brongniart, Mantellia. 



J 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. 



Mantell, p. 387. 

 124 



