56 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. I. 



TREES AND CYCADEOUS PLANTS OF PORTLAND. Case F. 

 In this Case, and on the top of the same, are many subcom- 

 pressed, spheroidal, and sub-cylindrical silicified bodies, hav- 

 ing the surface covered with lozenge-shaped scales ; these are 

 fossil plants closely allied to the recent Zamiee, and were 

 obtained from a remarkable stratum in the Isle of Portland, 

 named the dirt-bed, which occurs in the quarries on the north 

 of the island, a few feet above the layer of building-stone for 

 which Portland has so long been celebrated. These fossils are 

 found associated with the erect stems and prostrate trunks 

 and branches of large coniferous trees, of which there is an 

 example twelve feet long on the top of Case D. The circum- 

 stances under which these petrified trees and plants occur are 

 so extraordinary, as to warrant a brief notice of the pheno- 

 menon in this place. 



The Isle of Portland is a bold headland to the south of 

 Weymouth, about four and a half miles in length and two in 

 breadth, and is united to the mainland by a bar of shingle, 

 called the Chesil Bank. It presents on its northern aspect 

 a precipitous escarpment about three hundred feet high ; and, 

 declining towards the south, appears when viewed from the 

 east or west, as an inclined plane rising abruptly from the 

 sea. The base of the island consists of Kimmeridge clay, 

 which is surmounted by beds of sand and thick layers of the 

 oolitic limestone or Portland-stone. The strata dip to the 

 south at an angle corresponding with the outline of the surface. 

 The coasts are steep ; the base of Kimmeridge clay forming 

 a talus surmounted by perpendicular crags of oolite. The 

 southern extremity consists of low limestone cliffs, which are 

 worn into numerous caverns by the constant action of the 

 waves. 



The summit of the northern brow, to a depth of about 

 thirty feet, is composed of beds of laminated calcareous shale, 

 locally termed "the Gap ;" and sections of these strata are 

 exposed in the quarries that are opened for the extraction of 

 the building-stone which lies beneath. 



Immediately upon the uppermost bed of limestone, which 

 is a coarse rock, full of cavities and imprints left by the decay 

 of the usual species of marine univalve and bivalve shells of 

 the Oolite, are layers of calcareous shale a few feet in thick- 

 ness, in which no vestiges of marine fossils have been observed; 



