ROOM I. PETRIFIED FOREST OF PORTLAND. 57 



and whose laminated structure, and the presence of horizontal 

 seams of carbonaceous earthy matter, with interspersions of 

 vegetable remains, indicate a fluviatile or fresh-water origin. 

 Upon these deposits is a layer, from one to two feet thick, of 

 a dark brown friable loam abounding in lignite, and so similar 

 in appearance to common vegetable earth or mould, as to have 

 acquired the name of dirt-bed from the quarrymen. In and 

 upon this bed are numerous petrified stems and branches of 

 coniferous trees, and plants allied to the Zamiae. Many of 

 the trees and plants are standing erect, as if petrified while 

 growing on the spot ; the trunks of the trees extending 

 upwards into the limestone above, and vestiges of the roots 

 being traceable into the dirt-bed. The upright stems are in 

 general a few feet apart, and but three or four feet high, and 

 are broken and splintered at the top as if they had been 

 wrenched off at a few feet from the ground. They are from 

 a few inches to three or four feet in diameter; portions of 

 prostrate trunks have been collected, indicating a total height 

 of the originals of thirty or forty feet. In many instances 

 fragments of branches remain attached to the stem. The 

 cycadeous plants occur in the intervals between the upright 

 trees, and the dirt-bed is so little consolidated that specimens, 

 evidently standing in the position in .which they originally 

 grew, may be dug up with a spade. The strata above the dirt- 

 bed consists of finely laminated cream-coloiired shaly lime- 

 stone, in which casts of the fresh-water crustaceans (Gyprides) 

 so abundant in the Wealden, are the only organic remains 

 hitherto noticed. These deposits are covered by the modern 

 vegetable soil, which but little exceeds in depth the ancient 

 one above described, and instead of supporting cycadese and 

 pine -forests, barely maintains a scanty vegetation. Here, 

 then, we have the remains of a petrified forest of the ancient 

 world, the trees and plants, like the inhabitants of the city in 

 Arabian fable, being changed into stone, yet still retaining the 

 places they occupied when alive. 1 



MANTELLIA (M. nidiformis and M. cylindrica). Case F. 

 Such are the remarkable conditions under which the fossil 

 cycadeous plants named Mantellia, by M. Ad. Brongniart 



1 For geological details see "Wonders of Geology," 6th edit. p. 385; 

 or, "Geology of the Isle of Wight," 2d edit. p. 393. 



