Hills of Igneous Rocks. 291 



grew, and from this it is plain tffat the conditions 

 attending the deposition of the Carboniferous strata, 

 were in great part terrestrial. 



In the Scottish Coal-measures there are in Edin- 

 burghshire over 3,000 feet of coal-bearing strata, so that 

 the lowest bed of coal may be nearly three thousand feet 

 below the highest bed, in the centre of the basin, where 

 the strata are thickest. Most of the beds rise, or * crop,' 

 as miners term it, to the surface somewhere or other, 

 this ' outcrop ' being the result of disturbance of the 

 strata and subsequent denudation, and it is by means 

 of this disturbance and denudation that we are enabled, 

 by an easy method, to estimate the thickness of the 

 whole mass of strata, and to prove that one bed lies 

 several thousands of feet below another. 



In the Scottish area, during the formation of part 

 of the Old Red Sandstone and of the Coal-measures, 

 many volcanoes were at work ; and thus we have dykes 

 and bosses of felspathic trap and greenstone, and 

 interstratifications of old lava streams, and beds of 

 volcanic ashes mingled with common sedimentary strata. 

 These, being often harder than the sandstones and shales 

 with which they are interbedded, have 'more strongly 

 resisted denudation, and now stand out in hilly ranges, 

 like the Pentland, Ochil, and Campsie Hills, the 

 Renfrewshire and Ayrshire Hills on the Clyde, or in 

 craggy lines and bosses, like Salisbury Crags, the 

 Lomond s of Fife, and the Garlton Hills in Haddington- 

 shire, which give great diversity to the scenery, without 

 ever rising to the dignity of mountains. 



Having thus briefly rehearsed the mode of formation 

 of the more important Scottish formations, we may al- 

 ready begin to perceive what is the cause of the moun- 



u 2 



