MUSEUM OF COMPARATIVE ZOOLOGY. 189 



Sound distiuct proof, afforded by the numerous glacial scratches, that 

 the ice turned sharply to the east through about the same angle as 

 would be necessary to lead the train we are considering to the western 

 end of Martha's Vineyard. The same eastward turning is observed in 

 the direction of the glacial scratches at the west end of the mountain 

 range of Mt. Desert, where a change of over fifty degrees of arc in 

 the path followed by the ice evidently took place within a north and 

 south distance of about one thousand feet.* 



A glance at the map which accompanies this report will make it evi- 

 dent that by far the larger part of this boulder train lies beneath the 

 level of the sea. At least nine tenths of its area is so hidden from view. 

 Fortunately for the inquirer, a considerable, and perhaps the most impor- 

 tant, part of the evidence which it affords remains open to inspection. 



Description of the Source of the Train at Iron Hill. 



The conditions of the deposit whence the boulder train was derived 

 deserve special attention. They are eminently peculiar, and singularly 

 well suited to be the subject of an inquiry such as is proposed in this 

 writing. The material of which the Iron Hill is formed is a peridotite, 

 a phase of olivine gahbro. The percentage of metallic iron in the ore 

 varies between 25 and 45 per cent. The remainder of the mass con- 

 sists in the main of olivine and felspar : the olivine is in small rounded 

 grains, but the felspar generally occurs in the shape of considerable 

 crystals. These are irregularly scattered through the mass, appearing 

 in the greater part of the deposit : they constitute a very striking 

 feature in the rocks, and enable the observer with great certainty to 

 recognize a fragment derived from it. Under the influence of the 

 weather, the surface of the boulders derived from this locality assumes 

 a dark rusty brown color, while the felspar crystals retain a whitish hue. 

 Pebbles of this material, on account of their conspicuous features, are 

 often visible at a surprising distance, and present such peculiarities 

 that the trained eye will never mistake them for those formed from 

 other deposits. 



The area occupied by the peridotite deposit of Iron Hill is singularly 

 limited. It appears as a unique boss of rock rising from the tolerably 

 level country about it to the height of sixty feet above the base. (See 

 Plates I., II., and III.) The greatest length of the mass is about 1,200 



* See Report on the Geology of Mt. Desert, Eighth Annual Report of the Di- 

 rector of the U. S. Geological. Survey, p. 1004. 



