384 



THE WONDERS OF GEOLOGY. 



Lect. IV. 



300 feet high ; it is united to the main land by a bank 

 of shingle called the Chesil beach. It presents, on the 

 north, a precipitous escarpment, and declining towards the 

 south, appears, on approaching it from the east or west, like 

 # an insulated inclined plane, rising 

 I abruptly from the sea. The southern 

 extremity is flanked by low calcareous 

 cliffs, which, from the constant action 

 of the sea, are worn into hollows and 

 j caverns. The base of the island is 

 I formed of a blue clay (Kimmeridge 

 t clay), surmounted by beds of sand; 

 p and on these are superimposed thick 

 I strata of the oolitic limestone, known 

 , as the Portland stone, which is exten- 

 , sively quarried in the northern brow 

 \ of the island. 



The strata dip to the south at an 

 \ angle which corresponds with the pro- 

 i file of the island. The coasts are steep, 

 ! the base of Kimmeridge clay forming 

 I a talus, surmounted by perpendicular 

 crags of oolite. The southern extre- 

 mity consists of low oolitic limestone 

 cliffs, which are worn into numerous 

 caverns by the force of the waves. 



The northern brow of the island, 

 to a depth of upwards of twenty feet, 

 is formed of finely laminated fresh- 

 water limestone, locally termed " The 

 Cap" which belongs to the Purbeck 

 system, and is superimposed on the 

 uppermost bed of oolite. "The Kimmeridge clay, which is 

 the lowest visible deposit in the island, contains the usual 

 marine shells, &c. that prevail in this bed in the Isle of 



