SHIANT ISLES. GEOLOGY. 441 



mass, having the aspect of black chalk when broken, and 

 not much more hard : it also contains belemnites sparingly 

 dispersed. The indurated clay most commonly occupies 

 the uppermost place, but that rule is not absolute. This 

 is of a blueish or of a purplish grey colour, with a ten- 

 dency to split in a direction parallel to the beds ; and 

 is so soft as to be scratched by the nail. The siliceous 

 schist is dispersed through these beds in a very irregular 

 manner, in small fragments and nodules, or in thin por- 

 tions of discontinuous strata. It will presently be seen 

 that it forms a more conspicuous object on the east side 

 of the island. 



The most remarkable variety of the shale is charac- 

 terized by a feature very prevalent throughout these beds 

 and abounding equally in the siliceous schist. It some- 

 times consists of an aggregated mass of globular grains, 

 resembling such a substance as would result from cement- 

 ing together a quantity of mustard seed ; in other cases 

 it is like a mass of damaged gunpowder in which the 

 grains are not obliterated. These grains vary from the 

 size now mentioned to that of a pea; but any one bed 

 is always composed of grains of the same, or nearly equal, 

 sizes. When fresh broken this structure is less apparent, 

 because the intervals are filled with a looser substance 

 of the same argillaceous nature, which when washed out 

 by the action of the rains or the sea, leaves the exposed 

 parts resembling, as above mentioned, a stratum of distinct 

 or slightly aggregated spherules or grains. In a few 

 instances the intervals are filled with a soft greenish-brown 

 steatite, similar to that which, in Sky, is so common 

 among the trap. I have met with no account of such 

 a rock, but have found other instances of it among the 

 strata of siliceous schist which occur in the trap rocks 

 of Sky. In what respect such a disposition may have 

 been influenced by the vicinity of the trap it is impossible 

 to conjecture ; yet, if this had been the cause, we should 

 have expected to find the whole of the beds possessed 



