SURFACE GEOLOGY. 13 



of the Geological Survey of Ohio for the same year. A summary of the 

 facts cited in these papers, with others observed since, is given below. 



Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario occupy 

 basins excavated by mechanical agencies in undisturbed and nearly 

 horizontal sedimentary rocks. Of these, Lake Michigan is 900 feet deep, 

 with a surface level of 578 feet above tide ; Lake Huron is 800 feet deep, 

 with a surface level of 574 feet ; Lake Erie is 234 feet deep, with a sur- 

 face level of 565 feet ; Lake Ontario is 450 feet deep, with a surface level 

 of 274 feet above the sea. An old, excavated, and now filled channel 

 connects the basins of Lake Huron and Lake Erie. At Detroit the rock 

 surface is 130 feet below the city. In the oil regions of Enniskillen and 

 Bothwell, on the opposite side of Detroit river, from 50 to 200 feet of 

 clay overlie the rock, where the land surface is but little above the level 

 of Lake Huron. What the greatest depth of this channel is, is un- 

 known. 



A low, area over which no rock is found, apparently deeply underlaid 

 by gravel and sand, stretches across from Lake Superior, east of the 

 Grand Sable, to Lake Michigan. This probably marks the line of deep 

 channel once connecting the basins of these two lakes. (Winchell.) 



An excavated trough runs northward from Lake Michigan to the north 

 line of Iroquois county, Illinois ; thence south-west through Cham- 

 paign county, beyond which point it has not been traced. Its western 

 margin is sharply marked at Chatsworth, Livingston county, where it 

 has a depth of 200 feet, and reaches to the Cincinnati group. Further 

 north its bounding walls are composed of Niagara limestone, and termi- 

 nate in buried cliffs on the Calumet and Kankakee rivers. At Bloom- 

 ington this trough has a depth of 230 feet, and it there contains one or 

 more strata of carbonaceous earth, supposed to represent ancient soils. 

 Where penetrated in other localities, the depth of this channel is from 

 75 to 200 feet. — (P. H. Bradley.) In the excavations for the piers of the 

 new bridge at St. Louis, rock was reached at a depth of 100 feet below 

 the surface of the stream, o'n the margin of the old channel. Its central 

 depth has not been detemined. The Ohio throughout its entire course 

 runs in a valley which has been cut nowhere less than 150 feet below 

 the present level of the river. At the junction of the Anderson with 

 the Ohio, in Indiana, a well was sunk 94 feet below the level of the Ohio 

 before rock was found. — (Hamilton Smith.) In the valley of Millcreek, 

 in the suburbs of Cincinnati, gravel and sand were penetrated to the 

 depth of 120 feet below the stream before reaching rock. On the mar- 

 gin of the Ohio, at Cincinnati, gravel and sand have been found to ex- 

 tend to a depth of over 100 feet below low-water mark, and the bottom of 



