OF THE EARTH'S SURFACE. 175 



" seems best to wait till the phenomenon is observed before 

 " we seek for the explanation of it." 



Now it appears to me, that these vast assemblages, in which 

 blocks of every size, and shape, and quality, some sharp, some 

 round, are confusedly scattered through clay, are inexplicable 

 by any diurnal cause, and do call for some particular solu- 

 tion. 



Such parts of the torrent as encountered less of the strata of 

 shale and clay, would hurry along with them the comminuted 

 sandstone, and deposite it in the form of sand and of gravel. 

 Vast accumulations have thus been formed in all our lower di- 

 stricts, the external figure of which, and of the clay, has acqui- 

 red, as we shall soon have occasion to point out, a character pe- 

 culiar to itself, and having externally a smoothness and regu- 

 larity, which forms a striking contrast with the abrupt and 

 most irregular dislocation which very commonly occurs in the 

 solid mass within. 



In the midst of this general wreck of all the frail parts, the 

 strongest masses, principally those which, like Arthur's Seat, 

 have been powerfully pervaded with whinstone, would resist 

 and defy all the impetuosity of the stream. The principles 

 which we have endeavoured to lay down, as to the influence of 

 firm obstacles on depositions and abrasions, would thus be 

 brought into action. 



The rock upon which the Castle of Edinburgh stands, toge- 

 ther with the site of the Old Town, exhibit the most perfect 

 example that could be conceived of the application of these 

 principles. The rock itself, about two hundred feet in height, 

 above its base, and bare on three-fourths of its circumference, 

 consists of one of the most complete and uniform masses of 

 whinstone that is known in this country. Its form is rudely 

 cylindrical, and from it the ridge upon which the Old Town 



stands, 



