SURFACE GEOLOGY. 195 
Lake Michigan, Lake Huron, Lake Erie, and Lake Onta- 
rio are basins excavated in undisturbed sedimentary rocks. 
Of these Lake Michigan is six hundred feet deep, with a 
surface level of five hundred and seventy-eight feet above 
tides; Lake Huron is five hundred feet deep, with a surface 
level of five hundred and seventy-four feet; Lake Erie is 
two hundred and four feet deep, with a surface level of five 
hundred and sixty-five feet; Lake Ontario is four hundred 
and fifty feet deep, with a surface level of two hundred and 
thirty-four feet above the sea. 
An old, excavated, now-filled channel connects Lake Erie 
and Lake Huron. At Detroit the rock surface is one hun- 
dred and thirty feet below the city. In the oil region of 
‘Bothwell, ete., from fifty to two hundred feet of clay overlie 
the rock. What the greatest depth of this channel is, is not 
known. 
An excavated trough runs south from Lake Michigan — 
filled with clay, sand, tree trunks, ete. — penetrated at 
Bloomington, Illinois, to the depth of two hundred and 
thirty feet. 
The -rock bottoms of the troughs of the Mississippi and 
Missouri, near their junction or below, have never been 
reached, but they are many feet, perhaps some hundreds, 
beneath the present stream-beds. 
The borings for oil in the valleys of the Western rivers 
have enabled me not only to demonstrate the existence of 
deeply buried channels of excavation, but in many cases to 
map thein out. Oil Creek flows from seventy-five to one 
hundred feet above its old channel, and that channel had 
sometimes vertical and even overhanging cliffs. The Beaver, 
at the junction of the Mahoning and Shenango, runs one 
hundred and fifty feet above the bottom of its old trough. 
The Ohio throughout its entire course runs in a valley 
Which has been eut nowhere less than one hundred and fifty 
feet below the present river. 
The Cuyahoga enters Lake Erie at Cleveland, more than 
