SKY. GEOLOGY. SECONDARY STRATA. 335 



it. In some parts the siliceous matter in question is 

 that variety of chert which occurs in limestones ; while 

 in others it approaches so nearly to common flint that 

 it can scarcely be distinguished except by the confused 

 nature of its fracture. This substance is in some places of 

 an obscure reddish or purple colour, more generally of 

 the various tints of grey found in common flint; while 

 in a few instances its aspect is such as nearly to re- 

 semble agate. Independently of this mineral, laminse of a 

 red schistose clay are found in some parts of the rock, 

 which in these cases becomes itself schistose and impure, 

 as if passing into a kind of calcareous or marly slate. 

 To these I may add compact felspar, which is found in 

 occasional masses or laminae among the cherty nodules, 

 but in one place only. This situation is singular, and not 

 at all consonant to the usual habits of that mineral.* 



If we may venture to speculate on the analogy of this 

 limestone to those already described, the resemblance 

 of the chert and red shale of the one, to the quartz and 

 hard schist, and to the sandstone and micaceous shale 

 of the other two, is sufficiently obvious : while there 

 is the same probability that the anomalies in its appear- 

 ance have been the result of the same causes which have 



* Tripoli occurs in this place, and under circumstances which explain, 

 at least one source, of a valuable mineral which* seems to be formed in 

 many different situations; and to be connected with rocks of very dif- 

 ferent characters, as far as the observations of mineralogists have hitherto 

 reached. It encrusts the surfaces of some of the limestones, varying 

 from half an inch to an inch or more in thickness, and being of a whitish 

 or pale ochry colour; while the rock itself is of a smoke grey. It is 

 evidently the result of decomposition ; although it is not easy to explain 

 how the action of the rains has washed away all the calcareous matter, 

 leaving nothing behind but the loose and finely porous mass of silex 

 which was once mixed with the limestone and now forms the mineral 

 in question. In similar circumstances the same substance occurs 

 abundantly near Diurness Bay in Sutherland, and the rotten stone of 

 Derbyshire appears to have an origin, virtually, if not apparently, the 

 same. 



