NORTHERN BOULDERS. 
333 
As we pass southwards over the higher groups, boulders become exceedingly rare; and 
finally toward the southern margin of the State, they are rarely seen. 
This approach to the southern limits of these northern masses corresponds with what is 
observed throughout the whole west, as far as the Mississippi. The drift containing northern 
boulders of granite is scarcely observable in the southern part of Ohio and Indiana ; and I am 
informed by Mr. Lawrence, of Aurora, Ja., who has travelled much in that part of the country 
on both sides of the Ohio, that the valley of this river seems to be a limit to the northern 
boulders, and they rarely (if ever) appear upon the south of it. Such facts are of the highest- 
interest, as enabling us to arrive at important conclusions regarding the means of transport of 
those huge blocks, which, over the whole continent, and even over the whole world, seem, at 
certain periods, to have poured down from the north in such immense numbers. Wherever 
any attention has been given to their southern extension, it has always been found confined 
within certain parallels, and we have no authentic account of northern boulders in intertropical 
regions.* 
The condition of the boulders in the Fourth District, is the same generally with these 
masses over every part of the country where they have been noticed. Some of them bear 
evidence of much wearing, being actually striated upon the surface, and sometimes flattened 
on one side, as if held in that position while moved over a bottom of gravel or sand resting 
upon the strata beneath.f For the most part, however, they bear no evidence of attrition be¬ 
yond what similar masses do a few miles from their parent rock, and thus offer no argument 
for their mode of transportation. I have met with many which are very angular, and with no 
appearance of attrition beyond what the weathering in their present situations would produce.]: 
Even if these boulders were all rounded, it furnishes no argument that they were worn into 
this form during their transportation from their original beds to places where we now find them. 
The process by which fragments of granite become rounded boulders, is illustrated by the 
desquamation which takes place in some granites, the weathering in place, and the attrition 
in mountain streams soon after leaving their native beds. In the mountainous region of 
* In Virginia and North-Carolina, I have seen rounded masses of hard granite and greenstone lying upon the surface, and 
having much the appearance of boulders. In every case where I examined these, I found them to consist of masses which had 
become rounded from weathering in place, and were fragments of beds or veins of a hard rock in a surrounding softer mass which 
had disintegrated. Prof. Rogers says he has found pebbles of granite in Tennessee. 
1 I attach very little importance to the supposition that boulders of granite have been worn smooth and striated upon one side 
while fixed in a floating mass of ice, and in that way worn down while rubbing over a stony bottom. Some boulders of this kind, 
which I have seen, are less than a foot in thickness, and two feet in length. Now is it possible that such,a boulder, having 
rounded edges, can be fixed in a mass of ice, so as to allow of such force being applied to it, without falling out, unless the pres¬ 
sure were constant?’ The beds of many of the streams in the granite regions of New-York are literally paved with boulders, 
which remain fixed in certain positions, while any fresh accumulation of stones and earth, with ice and water, pass over them, 
rendering the upper sides very smooth, while the lower may be little worn. Can such occurrences offer any explanation of this 
apparent polishing on one side by transportation? 
J There is a mass of greenstone in the town of Riga, Monroe county, which bears no marks of having been broken artificially, 
the angles of which are as perfect and unworn as a fragment just precipitated from a mass of the rock in place. On the road to 
Nunda, south of Mount Morris, there is a similar fragment of greenstone, which weighs several tons, and still the angles are very 
little worn. Numerous other examples might be cited, offering the same evidence as these. 
