284 GEOLOGY. 



Now, in places these sediments are even yet 200 or more feet in 

 thickness, so that it would be safe to estimate the average thickness 

 of the original deposits at 100 feet. A yearly increase of one- 

 sixteenth of an inch in thickness, would at this rate have required 

 19,200 years to form these deposits. This I consider a low estimate 

 for the length of the Loess age. 



Life of the Loess Age. 



At the close of this chapter will be found a list of the land and 

 fresh-water shells that I have found and identified in the Loess de- 

 posits. It will be seen that the list of land shells is quite large. 

 These, no doubt, were brought into this old lake during flood- 

 time. I have occasionally found large numbers of these shells 

 where drift-wood had evidently lodged and decayed. The fresh- 

 water and land shells are mainly such as are still to be found in the 

 same region, the exceptions being the prevalence of a large num- 

 ber of southern forms at one horizon of these deposits. As will be 

 seen, the species belong to quite a large number of genera. 



Occasionally I have found the teeth and a stray bone of fish, but 

 have not been able to identify any species. The remains of rab- 

 bits, gophers, otters, beavers, squirrels, deer, elk, and buffalo are 

 frequently found. Through the entire extent of these deposits are 

 many remains of mastodons and elephants, whose last vigorous life, 

 as Newberry remarks, expired in high northern latitudes. Lan- 

 caster County is specially rich in these proboscidian remains. They 

 are frequently found in this deposit in digging wells. In Lincoln 

 they have been found in at least twenty wells that have been dug 

 in and around the city. This town is near what appears to have 

 been the western shore-line of the Missouri lake of the period. Be- 

 tween it and the Blue River at Crete, there is a high divide 

 covered by Drift materials. These huge animals no doubt often 

 here came down to the shore to drink, and playing in the water be- 

 came mired in the mud. One tusk found in a well on P street, east 

 of Twelfth, must have been at least eleven feet long when entire. 

 It was so far decayed that it fell to pieces on exposure. 



For years I have been closely watching for human remains in 

 the Loess deposits. Eight years ago, three miles east of Sioux 

 City, Iowa, in a railroad cut I found a small arrow-head in these 

 deposits. I was looking for mollusks, and was digging after them 

 with a large knife when I struck something hard, and, laying it 



