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 distaut 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 Nevr England, where the firm set I'ocks are usually hard, and all 

 the debris 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. 



The Kate of Glacial Wearing at Iron Hill. 



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 suV)jected 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 Pi"ovidence 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 debris 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 000,000 squai'e 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 



