262 GEOLOGY. 



The following analysis of these alkaline deposits show how they 

 vary in character. Both specimens were obtained from Fillmore 

 County: 



Ino.i. 



NO. 2. 



Insoluble (silicious) matter 



21 00 

 1.17 



1.80 

 33.14 



11.33 

 3.80 



55.11 



Alumina 



79 



Peroxide of iron 



1 21 



Lime carbonate 



19.70 



Lime phosphate, undetermined 





Magnesia carbonate, " 



7.13- 



Potassa, " 





Soda, " 





Organic matter, " 





Moisture 



4.01 







These two specimens indicate the presence of a large quantity of 

 magnesia and lime. Along the Missouri the alkaline concretions 

 at this horizon are largely composed, in places, of magnesia. They 

 are white in color and vary in size from a pea to from one to three 

 inches in diameter. It is probable that a portion of the calcareous 

 materials that are present in these deposits came from the chalk 

 rocks of the Niobrara Group that still exists in northeast Nebraska 

 and Dakota Territory. I have sometimes found in the Drift, and 

 also mingled with other alkaline deposits, small chalk rock. At 

 one place below Plattsmouth one of these chips of chalk contained 

 a fish scale characteristic of the Niobrara Group. Among the 

 eroded calcareous materials that were carried down into this lake 

 the chalk rocks must have constituted a large portion. Evidently 

 the waters became supersaturated with alkaline matter either by 

 excessive activity of the eroding agents — ice and torrents — or, 

 which is more probable, by partial dessication of the lake. We 

 have an exemplification of this kind of agency in the present and 

 past history of the lakes in the Utah basin. The analysis of its 

 waters give a remarkably small per cent of carbonate of lime. 

 And yet the rivers bring a large amount of it annually into the 

 lake. King, however, has shown in the 40th parallel survey that 

 lime in the form of tufa or thinolite has been precipitated in im- 

 mense quantities during some portions of its history. When the 

 waters of Salt Lake, (Lake Bonneville formerly, King), receded 

 below the line of outflow, but were kept at a high level for long- 

 periods of time, great beds of tufa were deposited, especially along- 

 the shore, and to some extent towards its interior. Lake Lahontan 

 (west of Lake Bonneville) is still a more remarkable instance where 



