MUSEUM OF COMPARATIVE ZOOLOGY. 207 
are very resistant to erosion, while the given field is underlaid by soft 
deposits, we usually find the area thickly covered with boulders. This 
consideration will explain the very distant carriage of the erratics which 
we find in Southern Ohio, many of which have doubtless come from the 
region north of Lake Erie, or from points more than two hundred and 
fifty miles away from their present site, the whole of their course having 
been over rather incoherent strata. The reverse of these conditions 
exist in New England, where the firm set rocks are usually hard, and all 
the débris which comes in contact with them in the process of glacial 
transportation is apt to be worn to the state of minute fragments. In 
this part of the country we can rarely trace a glacial train for more 
than a score of miles. 
Tae Rate oF GuaciAL WEARING AT IRON HILL. 
é 
One of the most interesting series of observations which it proved 
possible to undertake in this inquiry concerned the rate of erosion which 
took place at Iron Hill during the time it was subjected to glaciation. 
Although, as will shortly be seen, the-results of this research are of a 
rather discrepant nature, certain general conclusions which appear to be 
of value were obtained. As will be seen from the map, the part of the 
trail which lies to the north of Providence has the form of an acute- 
angled triangle, of which the base may be assumed to have a length of 
about 20,000 feet and the sides an extent of about 75,000 feet. The 
area included in this field is about 750,000,000 square feet. Our aim 
is now to ascertain the amount of the débris from the hill which lies 
upon this surface. From a careful inspection of the ground, I came to 
the conclusion that it is reasonable to estimate the quantity of detritus 
from Iron Hill which lies within this area as equivalent to an inch in 
depth over its whole surface, or say 60,000,000 cubic feet. Now the 
visible part of the Iron Hill rock does not exceed 600,000 square feet. 
It may be prolonged to the southward beneath the detrital deposits for 
the distance of a few hundred feet, and may extend somewhat to the 
westward under the drift materials; but if we assign to the mass an 
area of 1,000,000 feet, we give it all the area which careful inquiry with 
‘the dipping needle shows to be probable. On this basis we may say 
that the train north of Providence contains the equivalent of a mass 
sixty feet in height which has been removed from the hill. 
Attention has already been called to the fact, that not more than one 
fifth of the eroded matter from Iron Hill went away in the form of 
