SUPERFICIAL DETRITUS. 
327 
At Rochester, on the other hand, the surface of the limestone is finely striated, and almost 
perfectly polished, by this abrading action. So evenly is the surface worn, that I have taken 
slabs a foot square, and laying the faces in contact, there is no perceptible difference in the 
planes. The material here resting upon the rock is fine sandy loam ; and in another locality, 
a mile farther south, it is covered by coarse gravel of limestone and sandstone pebbles, with 
boulders of granite. These slabs are, for the most part, too smooth to be illustrated in an 
engraving, but can readily be understood from comparison to a partially polished slab of 
marble. 
The surface of the Niagara limestone, which is covered with drift materials, is from two 
to seven miles wide, and extends the whole length of the district. In all localities where this 
rock has been exposed, I have observed these striae, in greater or less perfection, but in no 
other place so fine, or with the surfaces so evenly polished, as at Rochester. The example 
of this rock alone, shows how universal has been the operation of this agent.* 
The great force and powerful abrasive action is well illustrated in the accompanying plate, 
which is from a specimen obtained at Black-Rock, from the surface of the Corniferous lime¬ 
stone, and is completely covered with these grooves and striae. It presents some interesting 
phenomena, from the presence of little nodular concretions of hornstone, which, in some 
places, stand out from the general surface, having, from their harder nature, evidently acted 
as a barrier to the abrading force; and there is a long, elevated ridge of the stone upon the 
southern side of these, which was thus protected, as a pebble or a fixed stone in a running 
stream allows the accumulation of sand or mud beyond it. In one or two examples these 
nodules of hornstone have been broken off, and fracturing below the surface, have left a de¬ 
pression, which is partially smoothed out in the same manner as the surface. There cannot 
be a more instructive exhibition of this abrading force than is here presented, where the 
different degrees of hardness in the two substances have fully illustrated their respective 
power in resisting the force applied. The surface, when free from these nodules, is not a 
perfect plane, but appears in broad undulations, with the elevations and depressions parallel 
to the striae, and produced by the wearing of the surface. In this instance the direction of the 
grooves, with scarcely a single exception, is N. 35° E. and S. 35° W.t 
On the banks of the Niagara river, and also at Lockport and elsewhere, I have, in some 
instances, found the direction of these grooves variable, though having the same general 
course. At Lockport, however, I was informed by Mr. Barrett, that after clearing the earth 
from the rock, previous to excavating the Erie canal, one large groove was noticed, which 
was measured, and found to extend, without the least variation, more than one hundred feet. 
It will be remarked that the direction of these grooves corresponds very nearly with the 
direction of the great excavating force, and the deep valleys of the lakes and rivers of the 
*1 find in my notes the direction of the grooves at Rochester marked as IV.N.E. and S.S.W. with some slight variations, 
t See a communication on the subject of these grooved and polished surfaces, by George E. llayes and R. VV. Haskin, in 
the American Journal of Science. 
