PiCTURESQ-UE CHARACTERS OF ROCKS. 57 



the mountains of upper Lorn. In Fife and in 

 Perthshire, in the ridges of the Oehils and the Sidlaw, 

 it is utterly impossible to conjecture the place of any 

 one trap rock, from the outline or general aspect 

 of the ground. Yet the differences of the superin- 

 cumbent and subjacent rocks can not well be greater; 

 as the latter consist of argillaceous schist, of the 

 lowest red sandstone, of the lowest secondary lime- 

 stone, and of the coal series. Every possible variety 

 of outline, conical, undulating, or flat, will be found, 

 somewhere or other, attending the trap rocks of Scot- 

 land ; and he will be a fortunate geologist who is not 

 obliged to surround, and almost handle, every mass 

 of these rocks, before he can determine their nature 

 or assign their limits. The formidable cliffs of Saint 

 Kilda present a diversity of characters and aspects 

 differing from those of any other analogous rocks in 

 this country ; and, in Sky alone, there may be found 

 every variety of disposition and outline which is dis- 

 played in nature. Even the columnar form which 

 has been supposed to indicate basalt, is not limited, 

 either to that rock, or to the greenstones ; since it 

 exists in the claystones of Mull, Rum, and Arran, in 

 the Syenite of Ailsa, in the pitchstone of Egg, arid 

 even in the sandstone of Dunbar. 



If granite and the trap rocks, which have been sup- 

 posed to possess such marked and characteristic fea- 

 tures, are thus subject to variations in their pictu- 

 resque and general forms, still less is it possible to 

 rely on this guide in examining the various stratified 

 rocks of the primary and secondary classes. If in a 

 few situations, as in Coll, in Rona, and on the west 

 coast of Rossshire, gneiss may be recognised at a 

 distance by the insulated and naked grey rocks every 



