OF THE EARTH'S SURFACE. 173 



contrast is conspicuous in Salisbury Craig on the east, and in 

 Corstorphine Hill on the west of Edinburgh. Each consists 

 of a thick mass of whinstone, parallel to the strata beneath it, 

 which, in the first mentioned hill, dip rapidly to the east, and 

 in the second to the west. 



The surface of this district, together with the alluvial part 

 ofi ts mass, bears every mark of the effects which a wave of 

 sufficient magnitude to overwhelm it, might be expected to 

 occasion upon so multifarious an assemblage. 



Raised from below by the violent and abrupt means already 

 alluded to, in my last communication to the Society, this dis- 

 trict would present to a stream overwhelming it at any subse- 

 quent period, numberless points of attack. Many of the 

 rocks being rent in various ways, the hardest parts being in a 

 shivered state, would easily be carried forward. The soft 

 beds of shale or slate-clay being laid open to the attacks 

 of the current, would be deeply abraded by its action, 

 and thus masses, both, stratified and unstratified, that were 

 originally unbroken, would be undermined, and, yielding to 

 their own weight, would add to the quantity of moving 

 matter, and extend the field of attack upon the weaker 

 parts. The water would thus be loaded with a multitude of 

 blocks of every size, shape, and quality, and with a quantity of 

 clay, which being soon reduced to mud, through which these 

 stones were irregularly and confusedly scattered, would flow at 

 the bottom of the water, and along with it, and would be de- 

 posited, according to the laws already pointed out, when the 

 stream approached to a state of rest. Such seems to have been 

 the origin of that body of compact blue clay which forms a 

 material part of our low districts, bearing every indication of 

 having flowed as a mass into its present situation ; for it is to- 

 tally devoid of stratification, though frequently of great thick- 



ness« 



