The Pre-Glacial Valleys of Arran and Snoivdon. 149 



important additions to the geographical history of Arran as to form 

 a contribution of permanent value to the literature of Scottish 

 physiography. 



1. The Valley System of North Arran. 



North Arran has a comparatively simple structure in spite of 

 the great variety of its rocks. It consists of a nearly circular 

 block of granite (Fig. 1) which varies from 7 to 8 miles in diameter 



Brifish Milei, 



Fig. 1. — Sketch-map of North Arran, showing the faults (thick lines) and the 

 granite (within the broken line). 



and is surrounded by a girdle of metamorphic and sedimentary 

 rocks. They formerly covered the granite as a dome from which 

 rivers must have flowed radially in all directions. The highest 

 mountain in Arran, Goatfell, is 2,866 feet, and three other summits 

 rise over 2,600 feet ; hence the area was apparently cut down to 

 a plain at the height, in the granite district, of a little under 3,000 



