66 The American Geologist July, i897 
called Pimisi bay, (3) at the modern Des Epines rapids, and (i) at Mat- 
tawa. The falls were 25 to 30 feet high and the postglacial gorge made 
by them is very distinct. It is not quite half a mile long, but it is deep 
and averages only about 300 feet in width. The walls are of red granite 
and vertical 40 to 100 feet. A thin and highly inclined bed of crystal- 
line limestone, passing down into the gorge from the west, may have 
hastened the cutting somewhat. The ancient river was expanded to a 
lake inthe lake Talon basin, and made faint but distinct shorelines by 
wave action. One is 20 feet above the present lake and the other ten or 
twelve feet higher. The mark of the surface level of the river was 
quite plain at some of the rapids. On the north side at Des Epines rap- 
ids this mark is 55 feet above the present river. The channel at that 
level was between 6U0 and 700 feet wide and averaged .35 to 40 feet in 
depth, and the current was strong enough to move gravel and pebbles 
of small size. This corresponds in a general way with the size of the 
modern St. Clair river. 
Ancient rapids were recognized in three ways. There are several 
narrow passages that are heavily bowlder-paved. They mark the points 
where moraines cross the channel. At Des Epines and Mattawa the 
bowlders of gneiss and granite are worn and scoured into many curious 
forms. Many were found with basins or potholes bored in them and a 
few bored clear through so as to become ring-bowlders. At each of these 
rapids a stream enters just above and furnished a constant suj)ply of 
graval, tand and pebbles for the current to roll over and among the 
bowlders. The rapids below Turtle lake and Pimisi bay are of the same 
sort, except that the water issued from lakes, and so had no supply of 
gravel to scour with. The third way of recognizing rapids was by in- 
ference indirectly. Such rapids were in narrow detiles or canyons with 
walls of bare rock and the fact that rapids had existed there was infer- 
red from the observed drop in the surface level of the river above and 
below. The remains of the Nipissing-Mattawa river agree with the Ni- 
pissing beach in indicating that the Nipissing great lakes endured for 
a relatively long period of time. And so long as it lasted. Niagara had 
only the discharge of lake Erie. A detailed account of the scoured 
bowlders appeared in the March number of the American Journal of 
Science. "F. B. Taylor. 
"The Term Pecatonica Limestone." In his article on the "No- 
menclature of the Galena and Maquoketa Series," in the May number 
of this journal, Dr. P. W. Sardeson refers, in somewhat sarcastic lan- 
guage, to my application of the term "Pecatonica limestone" to that 
portion of the stratigraphic series of northwestern Illinois which is des- 
ignated the "Buff limestone" in the reports of the Illinois geological 
survey. He remarks of it: "A new name the use of which scarcely ac- 
cords vv^ith scientific usage, since that author neither defines the term 
Pecatonica limestone nor seemed to have any reason for introducing a 
new name. He was discussing an entirely distinct subject and so far 
as I can learn, he had no more occasion to introduce a new name for a 
