ALUM SHALE. 135 



are highly pyritous, others strongly bituminous ; some of them abound 

 with organic remains, others contain few or none ; some are inerusted 

 with the cone-in-cone coralloid, others are septaria. 



The greater part of these flat masses are composed of a hard 

 blue limestone, similar to what is called the lias limestone. Some 

 have even designated our aluminous strata the lias formation ; and the 

 lias is known to be interstratified with blue shale abounding with petri- 

 factions ; but though there is some analogy between our aluminous 

 beds and the shale of the lias strata, and both occupy a similar 

 position among the secondary strata, it would surely be improper to 

 give the name lias to the whole series, when we find here only a few 

 courses of imbedded nodules to which that designation can be strictly 

 applied. 



Where these masses of blue limestone are overflowed by the sea, 

 near low water, they are often perforated by shellfish of the pholas 

 family, who hollow out their little cells in this smooth fine grained 

 stone, as they alsa do in some of the harder beds of alum shale. 



In the rocky pavement of our shores, the limestone masses are 

 often seen elevated above the surface ; the shale, which is softer, be- 

 ing worn or washed away all around them. Where a regular course of 

 flat masses, like thin edged millstones, is thus exposed, it presents the 

 .singular appearance of a collection of little round eminences, each of 

 which resembles the base of a cone. A fine collection of these 

 protuberances, each having its millstone covering, may be seen on 

 the shore a little to the south of Peak ; and something of the same 

 kind occurs between Sandsend and Kettleness. 



Though most of the flat masses consist of a blue limestone, that 

 has a fine grain, and a smooth and somewhat conchoidal fracture, there 

 are others of a smoke grey colour, having a coarser texture, and a. 

 more dull and eai'thy aspect. The latter are remarkable far the un- 

 pleasant, and strongly bituminoU:S smell, which they emit when strupk 

 with the hammer. In the recent fracture, there ai'e a number of 



