The Columbia Formation in dST. W. Illinois. — Hershei/. 7 
The second figure of the head represents a lateral view of a 
specimen on which, by a fortunate coincidence or accident, 
the fossil has preserved for us another aspect. Here the teeth 
are equally well shown, and the truly cladodont form of the 
jaw is manifest by its exact reproduction of that of Cladodus 
magnificus. Here, however, the anterior portion is missing, 
and nothing has been preserved behind the jaws. 
Several other specimens of the same species are in the pos- 
session of the discoverer, but they at present add little to the 
details which have been furnished by the two remarkable ones 
here described, which have been excellently disengaged from 
the matrix by Dr. Clark. 
Explanation op Figures. 
Plate I. Head of Cladodus clarki, as shown on ventral aspect, flat- 
tened. 
Plate II, fig. 1. ,Side view of the head of Cladodus darld. from an- 
other specimen, left aspect. 
Plate II, fig 2. Riirht pectoral fin of Cladodus darki, dorsal view. 
THE COLUMBIA FORMATION IN NORTHWESTERN 
ILLINOIS.* 
By Osc-VR H. Hershev, Frt-epoit, 111, 
CONTENTS. 
Introduction. .. ., 7 
■ Florence Giavel S 
\'alley Loess ii 
Upland Loess 14 
Correlation with adjoining Glacial Drift 17 
Sequence of the Glacial H istory 20 
Correlation with the Columbia Formation in the Lower Mississippi 
\'alley 22 
Introduction. 
In a recent paper,f the writer has endeavored to show that 
the earliest known glaciation of the northern part of the state 
of Illinois long preceded the epoch of loess-depositing waters 
and was separated from it by an exceptionally pronounced 
period of subaerial erosion. This deglaciation interval is 
marked by the excavation of extensive valleys or gorges in 
the limestone bed rock of the region, valleys in the old drift 
*Ivead before Section E of the American Association for tin- Advance- 
ment of Science at the Brooklyn meeting', August 20, 18!>4. 
f'The Pleistocene Rock Gorges in Xorlhwesti'rn Illinois," Amehican 
Geologist, vol. xii, pi). .S14-323. November, IS'.KJ. 
