i 
12 SCIENCE. 
PECULIARITIES OF WEATHERING IN 
THE POTTSVILLE CONGLOMERATE. 
Tue striking characteristics of the Potts- 
ville conglomerate in eastern Pennsylvania 
are its highly siliceous composition and _ its 
solidity. Owing to a consequent great dura- 
bility, it stands out prominently along the 
Fig. 1.— Outcrop showing weathering along the plane of stratification. 
different mountain ridges which surround the 
anthracite coal-basins; but though, as com- 
pared with the associated rocks, its resistance 
to weathering is very great, the effects of this 
action are everywhere revealed on examina- 
tion. 
The surfaces of the finer and more compact 
varieties are frequently seen to be covered with 
numerous small holes, or pit-marks, resulting 
from the removal of separate grains. Blocks 
of the coarse pudding-stone have generally a 
very rough surface, the pebbles projecting half 
their thicknesses above the surrounding ma- 
trix ; and fragments of this rock are sometimes 
so thoroughly permeated and softened by per- 
colating water that they can be crushed to 
grains by the hand. 
Along the planes of stratification the sub- 
aerial decay of this rock is particularly well 
marked. Deep clefts and gashes are found 
along these planes, which frequently cut en- 
tirely across large masses, dividing them into 
separate slabs. -This action is best developed 
along the upturned edges of steeply inclined 
dips, where water has the best opportunity to 
accumulate and to prolong its action in incipi- 
ent grooves; and, with isolated blocks only 
slightly inclined, the increased decay along the 
upturned edges, due to this same cause, is often 
noticeable. A somewhat remarkable fact about 
such weathering is, that clefts parallel to the 
stratification are found in an apparently homo- 
geneous rock. In such cases a difference or 
deficiency of cementing-material must be the- 
directing cause. a | 
Weathering action across the plane of strati- 
fication is exhibited in its first stages by shal- 
low and narrow grooves, which run sinuously 
across ¢he rock. These have their origin in — 
little streams of rain- 
water which flow from 
the -surface down the 
sides of the rock. Once 
started, such a groove 
forms a channel whose 
drainage capacity con- 
stantly increases as the 
depression enlarges ; and 
by degrees the fine groove 
grows to a decided fis- 
sure, half a foot or more 
across, which the contin- 
ued action of rain-water 
cuts deeper and deeper 
into the rock. This fis- 
sure is generally of ap- 
proximately uniform 
breadth ; but, as it enters 
farther into the rock, the water drains into it 
from all sides, and an enlargement is some- 
times formed at the end, which I have seen to 
result in an almost circular hole, completely 
penetrating the rock. . 
The most peculiar and remarkable of all the 
results of this weathering action are, however, 
Her ha We | "y 
| A, ‘ Ny 
Sy ees 0) ae 
a\ ay ) Ye 
epi aN eae 
: I Ny 4 7 
Si ee 
Fia. 2. — Isolated conglomerate mass showing increase of weath- 
ering along the planes of stratification on the upturned edge. 
ew ae he 
those produced by a superficial action in the 
plane of stratification. Over flat surfaces of 
the rock, white, washed-looking patches oceur ; 
