§ 17. MIOCENE, OR MIDDLE TERTIARY. 225 



To complete this slight sketch of the geology of the district, we 

 may observe that there are some insulated patches of lacustrine 

 silt and clay, containing extinct mammalian remains, asso- 

 ciated with those of living indigenous species; these strata 

 appear to be post tertiary. 



The Coralline-crag has yielded to the researches of 

 Mr. Searles Wood and others, upwards of 400 species of 

 shells ; numerous corals ; teeth of several fishes of the 

 Shark family, some of large size ( Carcharias megalodoit) ; 

 teeth of Eagle-rays (Miliobatis) ; and ear-bones (otolithes*) 

 of the Sperm-whale, and other cetaceans. The corals 

 are of unknown genera, and the echinoderms, of which 

 there are sixteen species, are distinct from any in the 

 adjacent seas. 



From the Red Crag, nearly 300 species of shells have 

 been obtained, of which about one-half also occur in the 

 lower Crag. The number of extinct (or more properly, 

 unknown) species, is about 30 per cent, in the former, 

 and 20 per cent, in the latter group. Microscopic shells 

 of foraminifera are abundant. Terebratulae of great size 

 are often met with ; specimens six inches long have been 

 obtained by Mr. Charlesworth. 



The Mammaliferous Crag has evidently originated in 

 deposits of silt and sand, brought down from adjacent 

 land by a river, or streams of fresh water, into a creek 

 or bay in which marine detritus was accumulating. 

 With numerous marine shells, it contains some fluviatile 

 and terrestrial species ; and in it have been found teeth of 

 the Mastodon, Ursus spelseus, j and other contemporaneous 

 fossil mammalia. 



The uppermost beds appear to be entirely lacustrine or 

 fluviatile, for they contain fresh-water shells and bones 

 and teeth of land quadrupeds ; comprising those of field- 



* Medals of Creation, vol. ii. p. 603. 

 j Mag. Nat. Hist. vol. iii. p. 450. 



Q 



