292 LYELL'S ELEMENTS OF GEOLOGY. 



Age of the Granite of the Isle of Arran. 



the Firth of Clyde, on the west coast of Scotland, the four great 

 classes of rocks, the fossiliferous, volcanic, plutonic and meta- 

 morphic, are all conspicuously displayed within a very small 

 area, and with their peculiar characters strongly contrasted. In 

 the north of the island the granite rises to the height of nearly 

 3000 feet above the sea, terminating in mountainous peaks. On 

 the flanks 'of the same mountains are chloritic-schists, blue roof- 

 ing-slate, and other rocks of the metamorphic order (a), into 



Fig. 293. Section of Arran. 



B 



d d 



A, Crystalline, or metamorphic schist. e, Granite, 



c, Conglomerate, sandstone, limestone, and shale. D, Trap. 



which the granite (b) sends veins. These schists are highly 

 inclined. On their truncated edges rest unconformable beds of 

 conglomerate and sandstone, to which succeed various shales 

 and limestones, containing fossils of the carboniferous period. 

 All these secondary strata (c) are much tilted and inclined near 

 the hypogene rocks ; but are horizontal at a distance from them, 

 as in the south of Arran. Lastly, the volcanic rocks (d), con- 

 sisting of greenstone, pitchstone, claystone, porphyry, and other 

 varieties, traverse all the preceding formations, cutting through 

 the granite in dikes (tZ), as well as through the sandstone ; 

 which last they also overlie in dense masses, from 50 to 700 

 feet in thickness. 



Now as the different kinds of trap intersect all the other form- 

 ations, they are certainly the newest rocks in Arran. The red 

 sandstone and other secondary strata are older than the trap, 

 but newer than the metamorphic schists, for the red sandstone 

 conglomerates not only rest unconformably upon the schists, but 

 contain rounded pebbles of those crystalline strata. It is equally 

 certain that the schists are the oldest rocks in the island ; they 

 are more ancient than the trap and red sandstone, for reasons 

 already stated ; and the granite must be of newer origin, because 

 it penetrates them in veins. The only chronological point, 

 therefore, in which there can be any ambiguity, relates to the 

 plutonic formations. They are more modern, as before remarked, 



