MUSEUM OF COMPARATIVE ZOOLOGY. ag 
progressively diminish in numbers towards the bouudaries of the area 
over which they are scattered. Thus 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 that 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 channel 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 could thence 
readily be shipped by water to the furnaces along the Hudson and else- 
where. Moreover, next the 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 Greenwich, R. 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 about 1,000 feet for each mile of 
distance from the source, or in other words the lateral or excursive 
