290 F. H. Bradley—Geological Map of the United States, 
by the modern lakes and streams themselves ; while, if repre- 
sented, these and the beds of drift would, over much the larger 
part of the country, leave exposed only the narrow isolated 
outcrops of the earlier formations which the learner finds it so 
ifficult to correlate, in his mind’s eye, and would show next to 
nothing of the general structure which it is the main purpose 
of such a chart to exhibit. Still, charts of the Quaternary con- 
tinent would’ be exceedingly instructive, in their exhibition of 
basin of Middle Tennessee, its walls being mostly Subcarbon- 
iferous. Another occupied the western part of the Silurian 
near the tops of the “Knobs "* fully 400 feet above the Obio 
at Louisville,) and stretching far up the slopes of the Cincinnatl 
anticlinal on the east—the boundaries in this direction being #% 
yet unreported. A third occupied a large area on the flats of 
the Upper Silurian and Devonian areas of Northwestern In- 
diana ;+ and perhaps still others swept over the broad prairies 
of Illinois and the States further west. 
Dana has long since called attention to the now submerge! 
» 2 
es of 
old valley of the Hudson, as marked on the conieaee 
i 
m 
. i b 
great flood, direct to the deep water of the ~~ — by the 
p. 181. 
* Borden, Geological Survey of Indiana for 1873, 
+ Geology of Illinois, iv, 227. 
CS Oe OY Pease, he) Re ne eS a 
