ra 
PREBLE COUNTY. 413 
gradual thickening of the bowlders until we find them in the central 
part of Washington township so numerous as to render the tillage of the 
fields dificult. From this point the belt can be followed in a broad band 
to the south-eastward as far as to the county line and even beyond. Its 
length within the county will thus be seen to be at least ten miles. Its 
greatest breadth does not exceed three miles, but the east-and-west roads 
of the county cut across it diagonally so as to show sections of four or 
five miles in breadth. 
The bowlders range in size from one thousand cubic feet downwards. 
Of one hundred and two blocks that were lying on the surface within a 
small compass, the greatest length in any bowlder was seven feet. A 
second gave a measure of five feet; four exceeded four feet; six exceeded 
three feet ; thirty-five measured more than two feet, while the balance 
(fifty-five) were under two feet, none being counted that were less than 
one foot. It is probable that within the same area there were nearly as 
many more concealed by a shallow covering of soil. On the farm of 
David Potterf, near west Alexandria, 1,200 bowlders exceeding two feet 
in diameter were counted to the acre. There are points where they are 
certainly more numerous than this. The value of the land is dimin- 
ished where it is so thickly covered, the expense of raising and removing | 
them being considerable. 
The bowlders lie upon or very near to the surface. Numerous sections 
of the Drift beds in this district are furnished in the banks of streams 
and in artificial cuttings, but they do not disclose any unusual number 
of these blocks at any great depth. 
In their distribution they are irrespective of the elevations and irreg- 
ularities of the surface. They cover the high ground and the low im- 
partially. The central portions of the belt occupy a part of the great 
northern plain of the county, which has an altitude of about one thou- 
sand feet above the sea. 
There is a considerable variety of composition among them. The 
kinds most largely represented are named below, in the order of their 
abundance : 
1. Metamorphic slates, fine-grained, and very hard; in color, dark blue, red- 
dish, and green. 
2. Diorites, blue and green, frequently intersected by felspathic veins. 
3. Silicious conglomerates, excessively hard, whitish, light green, and purplish. 
4. Gneiss, two leading varieties, viz., orthoclasic or red-banded, and horn- 
blendic or black-banded. 
§. Granites of the ordinary types, many of the blocks intersected by felspathic 
veins, which often stand out in relief from the weathering surfaces. 
6. Porphyries, comparatively rare, some with white felspar crystals, but more 
with reddish crystals. 
