MUSEUM OF COMPARATIVE ZOOLOGY. 189 
Sound distinct 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 HI. 
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 gabbro. 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 thé 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. 
