Ch. XVII] 



THE BLACKDOWK BEDS. 



251 



Gault. It consists of a dark blue mar], sometimes intermixed with green- 

 sand. Many peculiar forms of cephalopoda, such as the Hamite (fig. 291) 



Fossils of the Upper Greensand. 

 Fie.2S9. 



Fig. 290. 



a. Terebratula lyra. ) Upper Greensand. 

 &. Same, seen in profile. J France. 



Fig. 291. 



Ammonites Hhotomagensis, 

 Upper Greensand. 



Samites spiniger (Fitton) ; near Folkstone. Gault. 



and Scaphite, "with other fossils, characterize this formation, -which, small 

 as is its thickness, can be traced by its organic remains to distant parts of 

 Europe, as, for example, to the Alps. 



The Blackdown beds in Dorsetshire, celebrated for containing many 

 species of fossils not found elsewhere, have been commonly referred to the 

 Upper Greensand, which they resemble in mineral character ; but Mr. 

 Sharpe has suggested, and apparently with reason, that they are rather 

 the equivalent of the Gault, and were probably formed on the shore of 

 the sea, in the deeper parts of which the fine mud called Gault was de- 

 posited. Several Blackdown species are common to the Lower cretaceous 

 series, as, for example, Trigonia caudata, fig. 299. We learn from M. 

 D'Archiac, that in France, at Mons, in the valley of the Loire, strata of 

 greensand occur of the same age as the Blackdown beds, and containing 

 many of the same fossils. They are also regarded as of littoral origin by 

 M. D'Archiac* 



The phosphate of lime, found near Farnham, in Surrey, in such abun- 

 dance as to be used largely by the agriculturist for fertilizing soils, occurs 

 exclusively, according to Mr. E. A. C. Austen, in the upper greensand 

 and gault. It is doubtless of animal origin, and partly coprolitic, prob- 

 ably derived from the excrement of fish. 



* Hist, des Progres de la Geol., <Szc., vol. iv. p. 360, 1851. 



