SHALE. 249 



sand, or both ; and, in this latter case, it sometimes 

 passes into flaggy sandstone. It seldom includes dis- 

 tinct fragments of other rocks; bat, in Sky, nodules 

 of trap are found in some of the beds. Some varie- 

 ties of shale, like others of primary argillaceous schist, 

 contain a decomposable pyrites; being, therefore, 

 wrought for alum, under the name of aluminous 

 slate. 



When in contact with trap, it sometimes passes 

 into siliceous schist, as elsewhere explained ; and, 

 under the same circumstances, often assumes a peculiar 

 concretionary structure, slightly noticed in the chapter 

 on that subject. In the Shiant Isles, in Arran, in 

 Fife, and elsewhere, it thus presents a granular ap- 

 pearance, resembling sometimes a mass of damaged 

 gunpowder, or an agglutinated heap of mustard-seed, 

 at others, of spheroidal concretions like pease. And 

 when the rock is indurated to the hardness of siliceous 

 schist, as is common, the softer intermediate parts 

 wear away by the action of the sea and air, so as to 

 produce a botryoidal surface. In rare cases, as in Fife, 

 such concretions attain the diameter of a foot, resem- 

 bling those of the sandstones in the same circumstances. 

 The columnar configuration occasionally found in 

 shale, is connected with its transition into ironstone 

 and into jasper; as noticed in the description of this 

 rock. 



The range of Geological position occupied by shale, 

 commences with the old red sandstone and reaches to 

 the chalk ; and it occurs also in the lacustral and 

 other tertiary deposits. Its proportion to that of the 

 accompanying strata is exceedingly various, while it is 

 commonly interstratified in thin laminae with them. 

 The tenuity of these is sometimes extreme ; and it is 

 rare to find considerable beds, uninterrupted by the 



