the Upper Lias and Marlstone of Yorkshire. 217 



The seams are sometimes 20 feet long by 6 feet broad, but they are often 

 smaller, and their greatest thickness is 3 inches. 



The Jet Rock is also peculiar for containing the remains of the Teleosau- 

 rus ; for though Ichthyosauri and Plesiosauri appear pretty equally distributed 

 through the upper shale, this (fluviatile ?) reptile is rarely found but in, or im- 

 mediately above, the Jet Rock. Its bones are generally scattered in separate 

 nodules, the upper jaw being scarcely ever united to the lower. With the 

 exception of the specimen in the Whitby Museum, I am not aware of any 

 instance of this animal having been found entire, whereas the other saurians 

 seldom want more than the head or paddles. 



On inspection of the section, it will be perceived, that the Jet Rock has its 

 peculiar suite of Ammonites, and here, as was before observed, A. heterophyl- 

 lus and A. jimbriatus make their first appearance, being about the size of 

 a half-crown, while in the " hard seam" they attain the enormous magnitude 

 of 1 and 2 feet in diameter. The Ammonites Crassulus, (n.s.) common to the 

 whole upper lias, on the other hand, has its greatest development in the Jet 

 Rock, gradually diminishing from 1^ inches to ^ an inch in the overlying beds. 

 An interesting, though perhaps it may appear trifling, circumstance attending 

 this fossil is, that as it decreases in size it becomes more numerous, many 

 hundreds being then found together. 



The lowest bed of the upper shale is hard, compact and sandy, and is sin- 

 gular for its great barrenness of fossils, occurring as it does immediately 

 beneath one so prolific as the Jet Rock. This scarcity, combined with the 

 arenaceous nature of the stratum, may perhaps afford some hints for elucida- 

 ting, hereafter, the anomaly of fluviatile reptiles being mixed up with pelagian 

 shells and fishes. It is a plausible speculation, that during the formation of 

 the upper lias, the bottom of the sea in which it was deposited, was gradually 

 settling. This conjecture is supported, by the greater prevalence of vegetable 

 matter and fluviatile (?) reptiles in the lower part, and the increase in num- 

 ber of the larger species of Ammonites, Nautili, and Belemnites in the upper. 

 This conjecture is further supported by the mineralogical nature of the 

 schistus, silex being more abundant in the inferior parts, and alumina in the 

 superior, an arrangement which would naturally take place, if the coast 

 during that epoch gradually sunk, and the land, the detritus of which afforded 

 the material for the lias, as gradually receded ; the finer argillaceous sedi- 

 ment being consequently carried into the deeper and more and more distant 

 parts. 



Marlstone. — On entering this series, we at once perceive a striking contrast 

 to the upper lias, for independently of the decided mineralogical difference, 



VOL. V. SECOND SERIES. 2 F 



