54 Prof. Sedgwick on the general Structure 



On the east side of Long Sleddale the beds of limestone are seen crossing- 

 a rivulet, about 200 yards above the houses called Little London ; and from 

 that place they may be traced up the southern branch of the same rivulet (called 

 Iron Crow Gill), for nearly half a mile, in a direction about north-east by east. 

 They then disappear under great accumulations of turf-bog, and alluvial mat- 

 ter, and are not again seen. Some peculiar felspathic rocks on which they 

 rest, and some thick beds of blue flagstone, by which they are surmounted, 

 may however be traced through a morass on the top of the mountain : there 

 can, therefore, be little doubt that the limestone beds are also continued past 

 the north side of a hill called Lord's Seat, and thence into the turf-bogs on 

 the east side of Yarlside Crag, to a place not more than a mile and a half 

 from Wastdale Head. 



Had this system of strata met with no interruption, it would, after a further 

 range of about four miles, have abutted against the zone of mountain lime- 

 stone. It is, however, cut off by a great boss of granite, which has deranged 

 the relative position, and changed the mineral structure of all the neighbour- 

 ing rocks : from which we may, I think, safely conclude, that the granite of 

 Wastdale Head (commonly called Shap granite), did not assume its present 

 position till some period after the formation of the transition limestone. 



I have been the more minute in these details because they enable us clearly 

 to establish a series of important facts in the geological history of the Cum- 

 brian mountains. 



First. They prove, that at a very ancient epoch, and probably during the 

 principal period of elevation, great cracks were formed, diverging from the 

 centre of the mountains, accompanied by great changes in the relative posi- 

 tion of the mineral masses on the opposite sides; and further, that these cracks 

 prepared the way for future valleys*. Now the principal valleys of these 

 mountains diverge towards all parts of the circumference, from a centre near 

 the high crests of Sca-Fell. Is it not, therefore, probable (though in the 

 absence of beds of limestone, like those above described, we cannot establish 

 the fact on direct evidence), that great lines of dislocation pass down the 

 greatest number of these valleys ? In making this supposition we merely 



* The usual appearance, on the opposite sides of the faults above described, is exactly that of 

 a great horizontal lateral movement, and is not, I believe, by any means entirely deceptive ; for 

 expansive forces of elevation acting on oblique planes, miglit easily produce such a movement. 

 The effect was probably of a compound kind. When beds are highly inclined, a mere subsidence 

 on one side of a fault will, however, produce on the surface the exact appearance of a horizontal 

 slide. It is not, therefore, always possible, especially in the absence of underground workings, to 

 determine the exact direction of the movements which have accompanied a fault. 



