134 Geology of Upper Illinois. 
Art. XII.—Geology of Upper Illinois; by Cuartes Upnam 
Sueparp, M. D., Professor of Chemistry in the Medical Col- 
lege of the State of South Carolina. 
Tue remarks contained in this memoir are derived from obser- 
vations made the past season, during a short residence at Rock- 
well, in La Salle county. Having entered Illinois by the way 
of Chicago from the northern lakes, I shall commence my obser- 
vations with some account of this place and its vicinity. 
Lake-shore near Chicago. 
The western shore of Lake Michigan, above Milwalky, pre- 
sents no rocks as seen from the lake, being generally level, and 
but little elevated above its surface. It consists either of a sandy 
beach, or, as is more commonly the case, of an abrupt bank of 
blue clay. Chicago is situated on a beach-shore; but the low 
ridge of sand which formerly intervened between its site and 
the lake, has in a great measure been obliterated, in order to im- 
prove the building lots contiguous to the water. The easterly 
gales, however, silt up fresh deposits of sand and gravel, which 
singularly enough for this secondary region, abound in grains of 
garnet, magnetic iron and epidote, as well as in pebbles of granite, 
gneiss, Sienite and trap. ‘These foreign materials are no doubt 
derived from primitive boulders scattered over the bottom of the 
lake. 
The city plat scarcely varies from a perfect level, and rises only 
high enough above the surface of the lake to secure it a bare im- 
munity from inundations during severe gales, and seasons of 
unusually high water. In the rear of the town lies a broad level 
tract of wet prairie, still lower than Chicago, being only about 
ten feet above Lake Michigan. The width of this tract varies 
from six to nine miles, while it extends as far down the lake as 
the unobstructed view can reach; and in an opposite direction, 
follows quite round to the head of the lake, where however it ex- 
periences an extraordinary modification from the presence of sand- 
ridges, which we shall presently describe. 
The origin of so extensive a region of lagoon, which is almost 
completely submerged during the spring freshets, is not easily 
accounted for; since it is not contiguous toa broad slope of coun- 
