498 



THE WONDERS OF GEOLOGY. Lect. V. 



form the principal lithological features of the calcareous 

 portion of the Oolite, and the uppermost deposits consist of 

 a series of such strata abounding in marine shells. From 

 the great employment of certain beds of this stone for 

 architectural purposes, and the extensive quarries that have 

 for centuries been worked in the Isle of Portland, this 

 upper group is called the Portland oolite. In the south of 

 England, as we have already had occasion to mention,* the 

 Portland beds are covered by the Weal den strata, and in 

 some places by layers of vegetable mould, and petrified 

 upright trunks of pine-trees. In the inland counties, as 

 Wiltshire, Berkshire, &c, they are overlaid by the lowest 

 member of the cretaceous formation, the Greensand, no 

 intercalations of fresh-water deposits being apparent ; but 

 the oolite limestone, full of terebratuhe, trigoniae, ammonites 

 and corals, is immediately covered by sand and sandstone, 

 containing other species of the same genera of marine shells. 

 The lower part of this group is composed of a bed of sand 

 {Portland sand) from 50 to 80 feet thick, which gradually 

 passes into the underlying clay. The fossils of the Portland 

 beds are very numerous ; large ammonites, pleurotomariae, 

 trigoniae, pectens, oysters, pernse, terebratulaa, turritellae, 

 &c. and bones of saurians, and drifted coniferous wood, 

 are among the prevailing organic remains. f 



11. The Kimmeridge clay. — This argillaceous deposit 

 consists of dark bluish and grey clay, which in some 

 parts passes into highly bituminous shale ; the name is 

 derived from Kimmeridge in Dorsetshire, where some of 

 the layers are sufficiently combustible to be used as fuel.J 



* See Geological Excursion round the Isles of Wight and Portland, 

 p. 393. 



f As a British locality exhibiting the Portland strata and their 

 characteristic fossils, Swindon in Wiltshire is the most interesting 

 with which I am acquainted. See Geological Excursions in my MedaU 

 of Creation, vol. ii. p. 9 2 7. 



X Kimmeridge Coal. See Excursions round the Isle of Wight, p. 3o7. 



