202 THE WONDERS OF GEOLOGY. Lect. III. 



accumulated; the entire series therefore constitutes, in geological 

 language, but one formation. With but a very few exceptions, no 

 vestiges of existing species of animals have been detected. 



2. the wealden FORMATION. — A series of fluviatile 

 deposits of great thickness and extent, with scarcely any 

 interpolations whatever of marine detritus. The litholo- 

 gical characters are, alternations of clays, limestones, sands, 

 and sandstones, with beds of freshwater shells and crusta- 

 ceans. The peculiar organic features are the abundance 

 of the remains of enormous land and aquatic reptiles, river 

 shells, and crustaceans ; and coniferous trees, cycadeous 

 plants, ferns, and other terrestrial vegetables. 



Observations. — This remarkable series presents the most unequivocal 

 example of an ancient delta, or assemblage of fluviatile deposits, hitherto 

 observed. The strata exhibit throughout the character of river 

 detritus, that has been accumulated in a vast inland lake, which 

 received the drainage of an extensive country, and had but a restricted 

 communication with the ocean. The Wealden extends over the whole 

 of the south-east of England, and a great part of the north of Germany. 



3. the Jurassic or oolitic formation. — A formation 

 of great thickness and extent, consisting of several groups of 

 marine limestones an delays, which abound in extinct marine 

 shells, corals, crinoidea, crustaceans, fishes, and reptiles; 

 with intercalations of fluvio-marine strata, in which are re- 

 mains of terrestrial reptiles, insects, and plants; and of two 

 genera of small extinct mammalia. Layers and beds of 

 coal, and lignite, of considerable extent, occur in some parts 

 of the system. 



4. the lias. — Marine clays, shales, and limestones full 

 of shells, crinoidea, crustaceans, fishes, and extinct reptiles; 

 with fluvio-marine beds, containing lignite, trunks and 

 leaves of coniferous trees, and cycadeous plants. 



Observations. — The Oolite and Lias, though conveniently distin- 

 guished by certain species of shells, &c, must be regarded as but one 

 natural series or group of deposits, formed in the bed of an ocean of 

 great extent, into which numerous streams transported the debris <>l 



