322 TtifEDALE ESCAEPMENTS ; 



1 



But if sandstone has the advantage of limestone so far as purely 

 chemical action is concerned, the non-porous character of the lat- 

 ter renders it much less liable to be attacked in the grain by 

 frost. How thoroughly even the most massive sandstones have 

 been permeated by water is shewn by the fact that their felspar- 

 constituent has been usually reduced throughout to the condition 

 of kaolin by abstraction of the potash in solution. Their porosity 

 is thus increased, and it seems probable that in the silent enemy 

 which creeps within their substance and tears it apart, they have 

 one no less deadly than carbonic acid is to limestone. 



Upon these limestones frost acts by breaking up the cuboidal 

 blocks, to which joints divide them, into angular flattened pieces, 

 which get scattered along the scarp. And amid the moist vege- 

 tation, rich in carbonic acid, they are probably more rapidly 

 dissolved than when in situ. I have said that mass for mass 

 escarpments of limestone do not seem to recede quicker than 

 sandstones. But an ordinary wall has no chance of resisting a 

 demolishing hand as long as the thickness of a peel-tower. Sel- 

 dom exceeding twenty-five feet, while the sandstones are some- 

 times three times as much, they shrink back towards the base of 

 thicker beds behind, and often become a subsidiary portion of a 

 sandstone feature, — a subdued ledge or knob in the talus-like 

 skirt of a crag. 



4. Shale. — Since shale tends to hollow out where associated in 

 vertical surfaces with sandstone and limestone, it would almost 

 seem superfluous to argue that in cropping to the general surface of 

 a country it should do so likewise. But as this fact, so elementary 

 to a student of physiography in Tynedale, has been disputed, it 



uuprotected surface having been dissolved away to that deptli since the glacial period. 

 Kinahan finds similar pedestals in Ireland to be seldom as much as three inches high, 

 except near the coast, where they are six inches. A thin cover of glacial clay may have 

 protected the rock for variable periods, until washed off. The brown unctuous clay, 

 often associated in. considerable quantity with Northumbrian Limestone, does not, I 

 believe, consist purely of the insoluble residue of dissolved rock. 



A curious limestone boulder, fantastically eaten into a network of holes, till it looks 

 ahnost like a Brobdignag fragment of a coarse-meshed FenesteUa, stands in the garden of 

 my friend, Mr. Coppin, at Bingfield. 



