SURFACE DEPOSITS. 105 



general government; and its origin rightly referred by him to 

 an accumulation of sediment in an ancient lake which was 

 afterward drained when its accnmnlated sediment became dry 

 land. By Professor Swallow, State Geologist of Missouri, 

 who examined the deposit in that State, where it is continuous 

 with the same in Iowa, it was given the name of "bluff." 

 This term, as a specific name for the deposit, is not unobjec- 

 tionable, but it is not easy to find one that every person would 

 consider to be better. Neither is it often expedient to change 

 established names, therefore, that of Professor Swallow is used 

 in this report; although it is thought that the term Lacustral 

 Deposit would have been better, as it is expressive of the mode 

 of its origin, and would accord with such accepted terms as 

 Alluvial, Diluvial, &c. 



The material of this deposit is perfectly homogeneous in 

 composition and uniform in color throughout, even where it is 

 two hundred feet thick. Specimens taken from the bluffs at 

 Sioux City, and compared with specimens from the bluffs upon 

 the southern boundary of the State, although nearly a hundred 

 and fifty miles apart, are not distinguishable from each other. 

 It is of a slightly yellowish ash color, except where darkened 

 by decaying vegetation; very fine and silicious but not sandy, 

 strictly speaking, for by the unassisted eye one can hardly 

 discover it in particles of granular silicious matter, so coarse 

 as to deserve the name of grains of sand. At the surface it 

 forms excellent soil, and if taken from a depth of two hundred 

 feet from the surface it proves to be equally fertile. Although 

 the material is so finely comminuted, it is not very cohesive 

 and not at all plastic, and the soil composed of it does not 

 "bake" or crack in drying, nearly so much as that which 

 contains an appreciable amount of clay in its composition. 



The following is in part the result of Prof. Emery's analysis 

 of a sample from Sioux City — a full account of which will be 

 found in his report on another page. 



Silica 82.15 



Iron 3.89 



Alumina 67 



Carbonate of lime 9.66 



14 



