= Ree ae ee (a SSD ET aia “ nA eTe AE eee 
. . 
520 GEOLOGICAL HISTORY OF THE GULF OF MEXICO. 
barrier elevations of the apres through which the river cut its valley 
during the Terrace epoch, as I have defined it for our region. I have been 
able to recognize there only ccd epochs of the Post- -tertiary period, 
nameiy, the Drift aad Terrace epochs; but, at the same time, I fully con- 
terraces I understand to be parts of flood-plains abandoned from time to 
time as the valleys were deepened by the action of the waters of their 
own streams, for I can see no evidence that they were produced, even in 
rt, by any aa or depressions of that part of the continent. At 
the close of the Drift epoch, we may assume that Iowa had a nearly 
level surface, aan as it does now, about eight hundred feet above 
the level of the sea, the highest point being about seventeen hundred 
feet above, and the lowest, the southeast corner, four hundred and forty- 
four feet above that level. The valleys have all been eroded out of this 
terraces are found in the Bluff deposit as well as in the altered Drift, show- 
ing that the Bluff deposit must have originated early in the Terrace epoch. 
The deposit was formed in a large lake-like expansion of the Missouri 
river, caused by a broad depression that was left in the ead of the 
Drift at the close of the Drift epoch, eth ee filled with the silt of the 
muddy stream that flowed into and from it, which, finally Aeneid its 
valley below, had only to sweep out a aan we the silt, which became t the 
Bluff deposit, leaving the terraces, as it did, in the Drift 
Professor A. WINCHELL felt prepared to hla fully Dr. Hilgard’s 
views in reference to the absolute continuity of the upper portion of the 
Drift deposits in the Northern and Bounin States. He had studied these 
deposits from the Gulf of Mexico to Lake Superior, and in the Southern 
States, especially along the Black Warrior and Alabama rivers, and had 
expressed the conviction to Prof. Tuomey as long ago as 1852, and 
ad published it in the ‘‘Cleaveland Proceedings of the Ass ociation ” in 
1853, that he found no grounds for assigning the semistratified deposits of. 
the Southern sede to a different age from the so-called ‘ altered Drift,” 
higher of the continent at tho same rt and by agencies of a 
seats pete 
Dr. G. Lirrie had studied this subject for a quarter of a century: 
When a small boy it was his custom with his pfaymates to “fight rocks 5” i 
‘avi 
alogy, he soon discovered that those pebbles which were clear and glass- 
like (Quartz) proved the hardest to break — they were sometimes half as 
