216 Mr. Hunton's Remarks on a Section of 



becomes gradually scarcer and smaller^ so that after a descent of 60 feet, it is 

 rarely met with, and I believe has never been found so low as the jet rock. 

 The Orbicula reflexa, and the small, but delicate and beautiful Plagiostoma 

 pectenoideum are associated with the Nucula, and similarly distributed. It 

 may be worthy of observation, that the Nucula ovum is singularly character- 

 istic of that portion of the upper shale used in the manufacture of alum, and 

 that wherever the shale occurs without this shell, it has been found unfit for 

 the remunerative production of the salt*. This is owing to the nature of the 

 shale, the upper part, where calcined, being richest in sulphate of alumina ; 

 not, as might be supposed, from containing originally more sulphur, but a less 

 proportion, the lower strata having such an excess of pyrites and bitumen, that 

 the contents of a kiln made from such schistus, invariably overroasts. A large 

 Pallustra, or Amphidesma, (the Amphidesma donaciforme of Phillips) is also 

 plentiful in the upper part; but it is occasionally found in the lower, and a 

 similar shell occurs in the Marlstone, 



But of all organic remains, the Ammonites afford the most beautiful illus- 

 tration of the subdivision of strata, for they appear to have been the least able, 

 of all the Lias genera, to conform to a change of external circumstances. 

 Of the species, most plentiful in the upper part, scarcely any are found in the 

 lower ; except a few doubtful individuals of A. communis (the prevailing spe- 

 cies in the " hard seam") which are now and then seen so low as the jet rock ; 

 and though A. Jimbriatus and A. heterophyllus are found in that stratum, 

 they are always dwarfish, seldom attaining three inches in diameter, whereas 

 in the upper beds, they are met with twenty inches in diameter. 



The portion of the series mentioned before as the "Jet Rock," is a very 

 compact and highly bituminous bed, from 20 to 30 feet thick and 150 

 feet below the surface of the formation. It contains many flattened, pyritous 

 nodules, which generally inclose organic remains. The rock itself is also 

 so very sulphureous, that when a small heap of it was calcined at the Loftus 

 works some time ago, the melted bitumen and sulphur flowed in flaming 

 streams. The whole of the jet used in the manufacture of ornaments, is ex- 

 tracted from this stratum ; for when occasionally met with higher up, it is like 

 coal in its fracture, and too brittle to work well ; and in the inferior strata, as 

 the marlstone, it forms merely a thin coat, enveloping a lump of fossil wood. 



* The Alum Works, of which lias shale is characterized by the Nucula ovum, are the Peak, 

 on the extreme east, and Stow Brow, Godeland Banks, Sandsend, Kettleness, Boulby, Loftus, 

 Selby Hogg, and Osnnotherly, on the extreme west. Failures have occurred to different works in 

 attempting the calcination of the lower portions of the Upper Lias, or those strata undistinguished 

 by the N. ovum. 



