380 
GEOLOGY OF THE FOURTH DISTRICT. 
Where there are frequent alternations of argillaceous and arenaceous strata, a series of 
falls and rapids are produced; but, in such cases, the total amount of wearing action is far 
less. The sketch above is an example of this kind, where the fall has scarcely receded from 
the lake shore. A slight indentation only is produced in the abruptly sloping bank of the 
lake, and the same outline is preserved as if, on either side, it were stripped of its earthy 
covering. The strata here are principally arenaceous, and offer little facility for rapid wear¬ 
ing away; while at Lodi falls, a few miles farther north, where the principal part of the rock 
excavated is shale, the fall is now about one mile from the lake. It is impossible to say, 
with certainty, that these streams began their operations at the same time; though, if such 
had been the case, they would never have receded at the same rate. 
183. 
Entrance of the Canasertnvlie into the Cattaraugus creek. 
In the more recently excavated channels we find the streams falling over the very edge of 
the cliff, having produced no perceptible recession in the margin of the fall. The sketch 
above is an example of this kind, where the Canaserowlie unites with the south branch of the 
Cattaraugus creek. The channel of this latter stream is one of very modern excavation, as 
might be shown from numerous facts, similar to those in relation to the Genesee about 
Portage. The cliffs are almost vertical, to the height of one or two hundred feet, and the 
gorge is narrow, presenting no deposits of drift. Its course is devious, however, and it some, 
times enters an ancient valley, presenting a very different aspect from the recent one. 
A series of observations, for one or two centuries, will enable us to speculate with some 
degree of certainty regarding the probable rate of recession in waterfalls. But the observa¬ 
tions require to be made at different points ; upon streams of different magnitude ; on falls of 
different elevations, and upon those falling over different kinds of rocks. The rate of reces¬ 
sion in one fall will be an unsafe guide for estimating that of all others. Accurate maps of the 
