MUSEUM OF COMPARATIVE ZOOLOGY. 197 



progressively dimiuish iu numbers towards the boundaries of the area 

 over which they are scattered. Tims in the more definitely indicated 

 portions of the train, say at a point five miles south of its origin, it is 

 possible, in favorable positions, to find a dozen or more fragments on 

 the surface of a square acre, while on the margins the average number 

 may not be one to ten acres of area. As indicated on the map, the 

 boundaries of the train denote the limits within which fragments have 

 been actually found. It is likely that occasional bits of the rock occur 

 at considerable distances to the east and west of the points where they 

 have been actually observed. From Iron Hill south to the city of 

 Providence, field observations indicate that the margin of the train has 

 been tolerably well defined. South of that point the boundaries become 

 constantly more and more obscure. 



The obscuration of the margins of the train between Providence 

 and the open sea is due in part to the fact tliat in the more southern 

 district the boulders are to a great extent worn out by the attrition to 

 which they have been subjected, but it is mainly attributable to the 

 fact that a large part of the trail is covered by the waters of Narragan- 

 sett Bay, or by the deposits of terrace gravels which have been formed 

 since the ice moved over this district. The only part of the boulder 

 train which is distinctly traceable in the region south of Providence 

 are the marginal portions of its field. The central axis of the accumu- 

 lation lies in the main chaiuiel of the bay. Not only is this southern 

 portion of the train to a great extent hidden beneath the sea, but the 

 portions of it which are exposed to view have been more exposed to 

 the interference of man than the section north of Providence. Along 

 the shores of Narragansett Bay the forests have been mostly cleared 

 away and the land brought under tillage. These conditions have been 

 favorable for gathering the boulders of iron ore, which conld thence 

 readily be shipped by water to the furnaces along the Hudson and else- 

 where. Moreover, next tlie shores, the pebbles of convenient size have 

 been much sought for boat ballast, for which use their great weight 

 admirably fits them. 



Trusting the indications given by occasional fragments found in 

 fields on either side of Narragansett Bay, the trail in the parallel of 

 East Greenw^ich, Pt. I., is about 27,000 feet wide, and near the mouth of 

 Narragansett Bay it has a width, as before remarked, of about 40,000 

 feet. Thus, in a distance of thirty-five miles, the average widening of 

 the path occupied by these boulders is abont 1,000 feet for each mile of 

 distance from the source, or in other words the lateral or excursive 



