X 
level of Lake Michigan. The solid rock beneath this region is shown by artesian- 
well borings to be about as follows: — 
Niagara Limestone 254 feet 
Hudson river (Cincinnati) 250 feet 
Trenton Lunestone 330 feet 
St. Peter’s Sandstone 155 feet 
Lower Magnesian 70 feet 
1059 feet. 
The culminating axis of elevation of these lower rocks, which is in Wisconsin 
in the vicinity of Madison and Devil’s Lake, is 700 to 850 feet above the lake. The 
St. Peter’s Sandstone is the source of the waters which come to the surface 
through artesian wells. The waters of the first well that was bored in Chicago, 
which was hi the vicinity of Western Avenue, rose to about 55 feet above the 
lake. The pressure which elevates the water, comes from the head in the dis- 
tant elevations of the rock, in Wisconsin. 
The surface rock in the northwestern part of the State of Illinois, is the 
Trenton (Galena) Limestone, which dips under the more recent rocks to the east 
and south. Detached particles of the Hudson river shales are found in the 
“Mounds,” and higher points overlying the Galena Limestone, in the northwest- 
ern portion of the State, but only the Niagara comes to the surface, in a broad 
strip along the western shore of the lake, from Milwaukee, southward to the 
southern part of Cook County. The lower beds of the Niagara are fine grained, 
buff colored, magnesian limestone, extensively quarried at Lemont, and very 
extensively used for building purposes in Chicago and vicinity. It is evenly 
bedded and easily worked. 
The upper beds, which alone appear at the surface in the region under con- 
sideration, are a gray, siliceous limestone. It is hard and not easily wrought 
into dimension stones, but it is made into lime and much used for road making. 
About forty feet of the upper portion within the limits of Chicago, contains large 
quantities of bitumen. Some of these beds are very rich in fossils. These have 
interested palaeontologists for several years past. Besides the species common 
to the Niagara in other places, many peculiar species have been described from 
this locality, showing that in these ancient seas, this area, which is now Chica- 
cago, had a fauna distinctly its own. Speaking in a general way, these rocks 
have a dip of from five to ten degrees towards the southeast or south; but there 
have been local disturbances, as at Stony Island, where the dip is as great as 
thirty-five degrees. Northward the rock does not appear at the surface outside 
the city limits. West and southwest of the city, it is near the surface over large 
areas, and is quarried in many places. 
It rises about twenty-five feet above the - lake within the city at Western 
Avenue, and in the bluffs on the sides of the Desplaines at Lamont these rocks 
are found more than six^y feet above the lake. This part of the basin of Lake 
Michigan is scooped out of the Niagara rock. 
Wherever the clay has protected the surface of the rock from action of the 
atmospheric agencies, glaciated surfaces are nicely preserved. They are now 
found planed and grooved precisely as the glaciers left them. The direction of 
these striae, is west of south, as indicated by the arrows upon the map. The ar- 
rows at Cheltenham Beach and those just north of the mouth of the Chicago river 
indicate the direction of the striae beneath the clay. Those along the canal feeder, 
whose direction is almost at right angles to that of the others, are placed there on 
the authority of Mr. Ossian Guthrie. We have not ourselves seen them, and 
cannot judge what may be the legitimate inferences from them. 
