56 KEY. PROF. G. F. WRIGHT, ON THE INFLUENCE OF THE 
But the volume of water is so tremendous and its fall so great 
that the largest masses of rock are moved by it, and, being 
rubbed together by the motion, are gradually reduced to 
powder, and carried away piecemeal, leaving the base of the fall 
unencumbered 
Underneath the Niagara shale there are four other persistent 
strata of alternate hard and soft character. The Clinton lime- 
stone is about 30 feet thick and very compact, hut it rests upon 
about 70 feet of slialey rock which is easily disintegrated. 
This Clinton shale in turn rests upon a stratum of compact 
Medina sandstone 20 to 30 feet thick ; and that upon a slialey 
rock reaching to the water’s edge. All these strata dip slightly 
to the south toward the cataract. Owing to this dip and to the 
gradient of the stream, all but the two upper strata disappear 
below the level of the stream a little more than half way to 
the cataract, so that practically our problem involves the simple 
question of the erosion of the 35,000 feet of the two Niagara 
strata. 
In 1841, Sir Charles Lyell visited Niagara, and from a hasty 
examination published a random guess that the rate of recession 
did not exceed 1 foot a year, and probably was not greater 
than 1 foot in three years ; according to which the beginning of 
the erosion of the gorge must have been as far back as 35,000 
years at least, and probably 100,000 years. Unfortunately, these 
figures have passed into the literature of the subject, and, owing 
to Lyell’s great authority, have been accepted as scientific facts. 
But Sir Charles was himself very far from regarding them so ; 
for, at the time, he urged Professor James Hall, of the New 
York State Geological Survey, who accompanied him, to make 
an accurate trigonometrical survey of the crest of the Falls, so 
that there should be a proper basis of comparison with future 
surveys, which would reveal the actual facts. 
Such a survey was made in 1842. Permanent monuments 
were erected at the points at which the angles were taken, and 
all the details properly recorded in the third volume of the 
report of the Natural History Survey of the State of New York. 
After the lapse of sixty-three years, the last of four recent official 
surveys of the falls was made in 1905 by Mr. AY. Carvel Hall. 
Taking these surveys as the basis of his calculations, Dr. G. K. 
Gilbert, one of the most experienced members of the United 
States Geological Survey,* has reached the conclusion that the 
actual annual rate of recession of the Horse Shoe Fall for the 
* See Bulletin of the U.S. Geological Survey, No. 306, 1907. 
