1887. | NEW YORK ACADEMY OF SCIENCES. 1% 
A year or two ago, while visiting friends in this region, I had 
heard the spot referred to as one of the local curiosities, but had 
no opportunity to examine it. This year, on returning to the 
same place, I determined to make personal inspection of the 
phenomenon. 
Parallel to the Pennsylvania Railroad, a little way south of 
it, runs the beautifully kept and planted road known as the 
‘«* Lancaster Pike;” this is crossed, some three miles west of Bryn 
Mawr, and between Villa Nova and Radnor, by the ‘Spring 
Mill” road, going southwest from Villa Nova station. <A few 
rods beyond this crossing, on the south side of the pike, the suc- 
cession of cultivated farms, grounds, and country seats is inter- 
rupted by a piece of young woodland, as thick and wild with 
flowers and undergrowth as though it were miles away from the 
dwellingsof men. On entering it, and forcing one’s way through 
the thickets, the reason soon becomes apparent for the state of 
nature in which this ground has remained. The wood is filled 
with boulders, many of them of great size, masses of rock in 
some cases fifteen feet across and seven or eight feet high. All 
are so weathered that their character is unrecognizable ; but on 
chipping away fragments, they seem to be generally alike, a 
dark fine-grained gneiss, with occasional thin seams of quartz.* 
So remarkable was the deposit that I was at first disposed to be- 
lieve that they must be boulders of decomposition ; but there is 
nothing similar in the neighborhood so far as I have examined. 
On the contrary, as a rule, the rock of the region exhibits very 
uniform and complete disintegration into soil. Nowhere could 
the ‘‘southern drift” type. of regular undisturbed passage 
from surface-earth by insensible gradation into stratified gneiss, 
be more clearly seen than at various points in this neighborhood, 
wherever exposed ; while weathered ledges or solid outcrops are 
scarcely to be found. Further west, indeed, among the Radnor 
Hills, and on the Valley Forge Hills to the north, rock exposures 
occur more freely. But on the other hand, occasional smaller 
boulders, unmistakably such, are seen here and there in the 
roadsides and fields around. 
The spot is locally known as the ‘‘ Field of Rocks,” and is 
certainly a remarkable feature. It was formerly, I am told, a 
favorite place for ‘picnics, etc., but of late yearsit is so grown up 
with young woods as to be almost inaccessible, and quite invisi- 
ble from the adjacent road. 
I am at a loss to frame a satisfactory theory as to these great 
' Dr. Britton, on examining the pieces presented, stated that the rock 
hoe perfectly familiar to him as the gneiss of the New York ‘‘ High- 
ands. 
