Geology of Upper Illinois, 135 
try, or one whose rapid descent’ might measurably compensate 
for want of area in giving rise to alluvial deposits. Neither do 
rivers of any magnitude find their outlet here, which, like the 
St. Clair where it enters Lake St. Clair, might produce flat plains 
of considerable extent. Its origin seems to have been connected 
with a higher level of the lake, when its waters advanced inland 
quite to the rolling prairie. Nor would this supposition be at all 
satisfactory perhaps, except for the knowledge we possess of the 
almost universal, rocky substratum which prevails over the wet 
prairie, coming for the most part to within a few feet of the top 
of the ground,—thus giving us the conditions of a hard bottom 
as forming the shore of the lake, upon which the sediment and 
wash of the coast was in the progress of ages spread out. The 
deposit covering this rocky floor, is a horizontally stratified blue 
clay, on top of which at Chicago, rests a yellowish clayey loam. 
Lake Michigun 
poll 
egrae 
On the subsidence of the lake to its present level, the beach- 
line in the region of Chicago must have begun to form. For a 
long distance up and down the lake, it is confined to one or two 
embankments; but on drawing near the head of the lake, by the 
way of the sap to Michigan city, we find the surface of the 
prairie invaded far inland by a succession of ancient beaches, 
cad. with the utmost regularity as to width and height, as 
well as conformity to the existing shore of the lake. I shall de- 
scribe them as they came into view on the stage road, endeavor- 
ing to robalek their character the more intelligible by means of the 
bove sk tructed from recollecti Leaving Chicago, the 
