50 BREAKS IN THE CONTINUITY OF STRATA BY VALLEYS. 



en masses and fragments into distant countries. The formation of 

 valleys constitutes an important subject of geological research: it 

 will be reserved for a subsequent part of the volume ; but it may 

 be useful to state to the geological student, that all stratified moun- 

 tains are only parts of extended strata, with which they were once 

 united. 



This will be more distinctly understood, by consuhing Plate IV. 

 fig. 1., which is intended to represent the general rise of the strata 

 from Sheffield in Yorkshire to Castleton in Derbyshire, intersected 

 by the valley through which the river Derwent flows. 



The town of Sheffield, fig. 1., is built over coal strata, which rise 

 towards the west, and disappear in that direction about five miles 

 from Sheffield (2). Here, the under rock makes it appearance (3), 

 which is a bed of coarse gritstone, more than one hundred and twen- 

 ty yards in thickness, forming the summits of all the mountains as 

 you advance to the vale of Derwent (4). The grit-rock rests upon 

 a thicker bed, of a different kind, composed chiefly of slaty sand- 

 stone, represented (5). On the western side of the valley, the grit- 

 rock (3) exists only as a cap or covering on Whin-Hill, a lofty moun- 

 tain, marked (6). Two miles farther west, the grit-rock disappears, 

 and the slaty sandstone, which is the base of Whin-Hill, forms the 

 summit of the celebrated Mam Tor, or the Shivering Mountain. 

 The mountain limestone (7) here makes its appearance as the base 

 of Mam Tor, and, farther west, the same limestone forms entire 

 mountains. The difference observable in the rocks east and west of 

 the Derwent, is owing to the general rise of the strata in the latter 

 direction. 



It is here obvious, that Whin-Hill, though it appears an isolated 

 mountain, is only a portion of the thick beds of grhstone, and slaty 

 sandstone, on the other side of the valley. 



It deserves notice, that isolated caps, like that on the top of Whin 

 Hill, fig. 1., (6.) often occur where we can trace no similar rocks in the 

 vicinity : they are, sometimes, the only remaining relics of a stratum 

 that has been destroyed, and removed by some of the great catas- 

 trophes that have changed the surface of the globe. 



When valleys take the same direction as that of a range of moun- 

 tains, they are called longitudinal valleys; when they cut through a 

 range of mountains, they are called transversal valleys: in the lat- 

 ter case, the strata on each side of the valley are genreally the 

 same. 



The small valleys which open into a larger valley, nearly at right 

 angles to it, are called lateral valleys. In some rare instances, a val- 

 ley is formed by the bending of the strata, which make a trough as 

 represented Plate I. Fig. 2. c. 



When considerable tracts of the upper strata are wanting, as be- 

 tween A, B, Plate I. Fig. 2., it is supposed that the lower strata have 

 been laid bare, by some convulsion that has torn off and carried away 



