THE MAKING OF MOUNTAINS 39 



of more or less horizontal " sheets " or " sills " or as 

 " dykes " and " veins." Traprain Law in East Lothian, 

 Salisbury Craigs at Edinburgh, and the Abbey Craig 

 near Stirling are examples of exposed intrusive sheets 

 or sills. 



Not far removed are the Bosses, huge igneous 

 masses, often granitic, which intruded into adjacent 

 rocks, and probably cooled and consolidated at greater 

 depths than those we have already mentioned. They 

 may appear on the surface through the more rapid 

 weathering away of the rocks into which they bulged. 

 Many granite mountains in Britain belong to this Boss 

 type, such as Goatfell in the island of Arran and the 

 dome-like Red Hills of Skye. The very different 

 peaked and pinnacled Coolin Hills are said to have 

 been carved out of a great boss of the rock called 

 "gabbro," representing an earlier intrusion than the 

 Red Hills. 



So far, then, what is clear ? The crust of the earth, 

 the lithosphere, consists of the more or less elevated, 

 but partly submerged, "continental plateau," and a 

 more depressed portion known as the " oceanic basin" 

 which descends in some places into enormous "deeps." 

 Some of these are six miles deep, so that if Mount 

 Everest could be thrown in not a trace of it would 

 be seen above the surface. Now the earth is still 

 cooling and contracting, as it has been doing for many 

 millions of years. This means that subsidence of the 

 crust goes on, both in the continental area and in the 

 oceanic basin and there is a good deal of evidence 

 that the meeting-line of the two great areas is a zone 

 of special instability, where mountains and troughs 

 are particularly likely to be formed. The marginal 



