250 SHALE. 



repetition of the sandstones, limestones, or clays 

 which it accompanies. The only exception known 

 to myself, is the sandstone series of Caithness and 

 Orkney ; which appears to form two parts of a com- 

 mon deposit. In Orkney, in particular, many exten- 

 sive tracts of shale occur, covering a great surface, and 

 reaching to so great a depth, uninterrupted by sand- 

 stone, that, as far as I can confide in observations on 

 a country so deeply covered with soil and peat, it 

 sometimes attains the thickness of seven or eight 

 hundred feet. Thus it here occupies a conspicuous 

 place among the secondary strata, and may claim the 

 rank of an independent rock. 



It is here alone that, in Britain at least, its geogra- 

 phical features can be studied ; since here only it is 

 unmixed with other rocks. These features are inva- 

 riably tame. The few hills are rounded, and unmarked 

 by protruding rocks or abrupt declivities ; while North 

 Ronaldsha and Sanda are nearly as flat as the sea 

 which surrounds them ; above which also they are so 

 little elevated, that it seems, in winter, to threaten their 

 existence. Yet in Westra, Rowsa, and the Mainland, 

 the abrupt vertical cliffs are more durable than those 

 of the sandstone in the same islands. 



The composition and texture of shale, with its 

 ready disintegration, point it out as the parent of 

 fertile and deep soils ; though, occurring so rarely 

 alone, there is little opportunity for judging of the 

 fact. In Caithness and Orkney, the covering of peat 

 is commonly so dense and deep, that the soil beneath 

 is as useless as it is inaccessible. Yet where, as in 

 North Ronaldsha, the surface is free of this, the ex- 

 cellence of the soil is indicated by a fertility which, in 

 such a climate, would scarcely be expected. 



The oeconornical uses of shale arc inconsiderable ; 



