AVGHZT.] OEIGm OF THE LACUSTRINE DEPOSITS. 251 



to disappear. This entire region became so depressed that the greater 

 part was submerged. How long this submergence lasted is an unsolved 

 problem. As the land in the course of ages emerged again from the 

 ■waters, under the influence of a milder climate, it gradually became 

 covered with a vast forest. The bed of this old forest is often struck in 

 digging wells in many parts of the West. It is often found in the gla- 

 cial Drift, and separates it into two portions. It is composed of black 

 soil, and where I have measured it, its thickness ranged from six inches 

 to three feet. It often contains partially decayed and partially or en- 

 tirely petrified wood. Over this old and now buried forest-bed the ele- 

 phant {Elephas americanus) and mastodon {Mastodon americanus) roamed 

 in company with the reindeer and musk-ox. A back molar of the left side 

 of the lower jaw of an elephant, obtained from this old bed in Saline 

 County, whichis in the university cabinet, measures seventeen inches from 

 front to rear. But the slow uj)ward movement of the land, accompanied 

 by a gradually-falling temperature, inaugurated a second advance of the 

 glaciers, which "wiped out" the forests that covered the land. This 

 period was followed by a still greater subsidence of the land toward the 

 north, when the glaciers began to disappear the second time. Accord- 

 ing to,Professor Newberry, who has profoundly studied this question, all 

 that region north of the Ohio, and westward beyond the Missouri, which 

 is now less than eleven hundred feet above the level of Lake Erie, was 

 covered with water. The depression was greatest toward the north, so 

 that in the east the AUeghanies, and their dependent foot-hills, and a 

 wide area of low country toward the south and west, formed a shore- 

 line to the interior sea of the period. This sea was often covered with 

 floating icebergs, which, melting, dropped their imbedded sand, gravel, 

 and bowlders to the bottom. Tbe old controversy concerning the method 

 by which the glacial Drift was formed had on both sides some elements 

 of truth. It was formed exclusively by neither glaciers nor icebergs, 

 but by both operating at different times in their own peculiar way. 



From this submergence the land slowly arose, and when the Missouri, 

 the Platte, and the Republican Elvers in their upper courses resumed 

 their work the Lacustrine age commenced. There is, of course, no 

 exact line where the one ends and the other begins, but it is safe to say 

 that the later centuries of this great subsidence witnessed the deposi- 

 tion of the Loess deposits. When it commenced, the greater part of Iowa 

 had become dry land. What was left of this great sea was tbe western 

 portion of Iowa, a- large portion of Nebraska, and the various lakes 

 along the Missouri, in the States through which it flows on its way to 

 the Gulf. Tbe Missouri, and sometimes the Platte, have been among the 

 muddiest streams in the world. If we go up the Missouri to its source, 

 and carefully examine the character of the deiDosits through which it 

 passes, we cannot be surprised at its character. These deposits being of 

 Tertiary and Cretaceous ages, are exceedingly friable and easy of disin- 

 tegration. The Tertiary, and especially the Pliocene Tertiary, is largely 

 siliceous, and the Cretaceous is both siliceous and calcareous. In tact, 

 in many places the Missouri and its tributaries flow directly over and 

 through tbe chalk-beds of thS Cretaceous deposits. From these beds 

 the Lacustrine deposits no doubt received their large per cent, of the 

 phosphates and carbonates of lime. Flowing through such deposits 

 for more than a thousand miles, the Missouri and its tributaries have 

 been gathering for vast ages that peculiar mud which filled up their 

 ancient lakes, and which distinguishes them even yet from most other 

 streams. Being anciently, as now, very rapid streams, as soon as they 

 emptied themselves into these great lakes, and their waters became 



