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Ei, Andrews on the Glacial Drift of Illinois. 75 
Art. XI.—Observations upon the Glacial Drift beneath the bed of 
— “bes as seen tn the Chicago Tunnel; by E. ANDREWS, 
.. M.D., Prof. of Surgery in Chicago Medical College. 
In the November number of this Journal there is an article 
from E. W. Hilgard, State Geologist of Mississippi, which con- 
tains serious errors ‘respecting the Drift formation in Llinois. 
The mistakes occurred, doubtless, because the distinguished au- 
thor had not an opportunity for personal observation here, an 
was obliged to rely on the information of others less competent 
than himself. 
He remarks that the erratic blocks are nearly all well water- 
worn, and that the drift “is more or less irregularly, but dis- 
tinetly, stratified,” and that “no glacier scorings are mentioned 
either on the pebbles or on the adjacent rocks.’ 
The truth is this. The drift of [linois, Indiana, Michigan and 
Wisconsin consists of two distinct formations, one above the 
other. The lower is the well known “ glacial drift” of authors. 
It is a heterogeneous mass, full of boulders and pebbles, which 
are only imperfectly rounded and sometimes quite sharp at 
the edges. Vast multitudes of the larger blocks are scratched 
and votes on one side by unmistakable glacier action. Ina 
large part of the formation it is extremely difficult to discover 
— traces of stratification, though it can be noticed by great care. 
cond formation always overlies the former, and consists 
of nae and gravel re vagy and Jilled with well 
rounded and water-worn pe e two its are so un- 
like each other that there is no peatilie of capiousiing them. 
Their relation to each other is best shown in the numerous rail- 
road cuts which traverse them. It is there seen that at a distance 
from the valleys of mee the old glacial drift usually comes 
to the surface, and often s into considerable eminences, In 
Illinois, at least, ne closest scrutiny generally fails to discover 
in it any stratifica As we approach the streams, however, 
the glacial drift ani out of sight, and is overlaid by perfectly 
well stratified sand an gravel. The latter exists in enormous 
quantities, sometimes bordering the valley miles in width, and 
by its thick masses fully maintains the general height of the 
country. At the border of the valleys of the streams the sand 
and gravel suddenly cease, terminating in an abrupt descent; or 
perhaps the lower strata may continue beneath the alluviam. of 
~ Lar lands. 
he Chicago tunnel is excavated for two miles in ws lacial 
arif beneath the bed of Lake Michigan. It has been elosely 
watched by a committee of the Chicago Academy of pa as 
well as by Mr. Chesborough the city engineer, Mr. Kroeshell the 
