Bt HUGH MILLER. 3213 



is worth while shortly to illustrate it. The decomposition of 

 shale is in a great measure the rationale of escarpments. 



There are situations, it must be allowed, in which shale is 

 more lasting than when exposed to disintegration in a bare scar. 

 In the waterfall in Dinley Burn, near Birtley, already alluded 

 to, a strip of shale, upon which the water splashes step after 

 step with considerable force, but which it protects from all but 

 very severe attacks of frost, stands out as a buttress, while on 

 either side are recesses, like the curves of a narrow B, where 

 spray and drip are heavy. The forcible erosion is evidently 

 less effective than the noiseless corroding waste. The same fact 

 may be remarked in the calcareous grit below the main fall of 

 Hareshaw Linn, near Bellingham. Among the furrows of our 

 escarpments shale is attacked rather by the quiet than the forcible 

 method. The impervious clay into which it softens allows fine 

 threads of water, that sandstone or limestone would at once ab- 

 sorb, to flow freely along, while its fine, often impalpable, tex- 

 ture supplies particles suited to so slight a conduit,, to which 

 a large grain of sandstone might be a boulder.^'' ]S"or can a cover 

 of grass prevent this invisible ''portage" by capillary streams. 

 liOng ago De Luc argued that after a slope sufficient to render a 

 country fertile for man's culture is attained, denudation must 

 cease. "According to the doctrine of this author," says Hutton, 

 in a vein of unwonted humour, '' our mountains of Tweeddale and 

 Teviotdale, being all covered with vegetation, are arrived at that 

 l)eriod in the course of things when they should be permanent. 

 But is it really so? Do they never waste ? Look at the rivers 

 in a flood : if these run clear this philosopher has reasoned right, 

 and I have lost my argument."! It is small change of position 

 to assert, upon the basis of Hutton' s argument, that rain water 

 cannot run off clay, even under grass, without becoming 



* Tlic hill-sides where they consist of clay are much more rurrowccl hy stream-courses 

 than where porous rocks form the surface, a fact very marked, as pointed out to mo by 

 Mr. B. N. Peach, among the shale and sandstone groups near Carter Fell. It is an Inter, 

 csling co;iiment on the conducting power of clays, that in the argillaceous gault regions 

 of 8. England, the bridges have to be made iciiUr than on the porous chalk. (Green's 

 Physical Geology.) 



t Theory of the Karlh, \o\. 11., (i. 204. 



