382 
GEOLOGY OF THE FOURTH DISTRICT. 
These speculations are offered, not with a view to any practical bearing, but to correct an 
erroneous impression which arises from the first view of these falls. Since there are now 
three falls, and since we suppose there was a period when only one existed, it is natural to 
infer that the same cause that first produced a separation would continue to operate to per¬ 
petuate the same condition. This would doubtless be true so long as the nature of the strata 
remained the same ; but it is equally evident that any change in these will change all the 
other conditions. 
195. 
Lower falls of the Genesee at Rochester. From a sketch by Mrs. Hai.l. 
The lower fall at Rochester has evidently receded but very slowly for a long period ; the 
broad expansion in the river below evidently indicates that the falling water and eddying cur¬ 
rents have done their work upon the cliffs for many centuries. Still within a short period the 
stream has excavated the rock deeper upon the west side, and during the dry season nearly 
all the water flows in that direction. This will eventually wear a narrow channel like the 
lower fall at Portage, leaving a platform the original bed of the stream on the east side. 
This locality offers a good exhibition of the succession of the strata. The Medina sand¬ 
stone rises to the top of the falls, and is succeeded by the Clinton group, and this again by 
the shale of the Niagara group. The layer of limestone containing the Pentamerus oblongus 
is here one of the most prominent and interesting portions of the mass, while in the succeed¬ 
ing shales occur the fossils figured under the Niagara group; thus presenting a succession 
of the strata, scarcely so well exposed in any part of the district. 
