GEOLOGY OF MOUNT DESERT. 65 



areas underlain by rocks of other kinds. Boulders from 

 the granite of the central belt are moved southward in 

 plenty over the surface of the bedded and volcanic rocks. 

 Not only so ; blocks of a coarse gray granite, easily known 

 by its large crystals of whitish feldspar, but not occurring 

 in the rocky structure of the island, are found here and 

 there over its surface. They come .from the mainland, 

 where this kind of granite is well known. One of these 

 boulders was to be seen, some years ago, close to the 

 summit of Green Mountain. Further than this, there are 

 fragments, generally of less than a foot in diameter, of a 

 fossiliferous shaly flagstone, sparingly distributed over the 

 western half of the Island ; these are easily identified as 

 belonging to a belt of Devonian strata some miles north- 

 ward from the mainland coast. No such rocks occur in 

 place on the Island. All of this peculiar transportation of 

 erratic boulders is ascribed to the ice sheet, aided in some 

 cases by its subglacial streams. The boulders are simply 

 the larger fragments that the ice sheet dragged along 

 beneath it, or carried in its lower portion. 



The unconsolidated drift by which the lower rocky floor 

 of the island is generally covered frequently possesses a 

 structure that gives still further indication of land-ice 

 action. Its lower part, lying close packed on the striated 

 bedrock, is a compact unstratified mass of stones, sand, 

 and clay ; the stones are of both local and distant origin, 

 being more worn and striated if from a distance, while 

 those that have been brought but a little way show fewer 

 signs of severe usage. Deposits of this kind are called by 

 the Scotch name, till. They are very generally spread over 

 the New England plateau, where they diminish the rugged- 

 ness of the rocky surface. Till also occurs in the valleys ; 

 but here it is often covered over by water-washed sands or 

 clays of somewhat later date. On the " Monadnocks," the 

 till is scanty : above five or six hundred feet, the Mount 



