151 



Last autumn, I received a parcel of " efflorescences," from 

 the soil of the Desert, between the head waters of the Missouri 

 River, bordering on the eastern flank of the Rocky Mountains, 

 from Lieut. Grover, U. S. A., who conducted a party engaged in 

 a reconnoissance of that region. The earth of many square 

 miles of surface is covered by a gray saline frosting, which 

 exists so abundantly, that vegetation does not appear, and the 

 features of sterility are so widely displayed, as to constitute a 

 true desert. 



Chemical analysis proved the efflorescence to be sulphate of 

 soda, with minute traces of sulphate of lime, and common salt, 

 mixed with fine sand, to which the gray color was due ; no other 

 saline compound was present. The sulphate of soda was nearly 

 anhydrous, being in fact Thenardite, but evidently formed from 

 a hydrous salt by dessication. No distinct crystals were included 

 in the specimen received. 



The occurrence of this salt, as an abundant exudation from 

 the soil, is interesting in a mineralogical view ; and its presence, 

 under the form of an efflorescence, illustrates the physical law, 

 in accordance with which saline waters rise through the soil, and 

 in evaporating from the surface, leave their saline parts at that 

 point, thus forming deserts. 



It has been long known to me, that the water of the Missouri 

 River contains sulphate of soda, and the presence of this salt in 

 the water occasions a change in the composition of the water of 

 the Mississippi River, after these waters become mixed, below 

 St. Louis. 



The water of the upper Mississippi contains a large amount 

 of organic matter, of a kind which, in changing its composition, 

 out of contact with air, attracts oxygen powerfully, and will de- 

 compose oxidized bodies. The dark color of the buried silt is 

 due to the presence of sulphuret of iron, formed from the oxide 

 of iron of an ochreous clay, reduced and rendered a sulphide 

 of iron, by the presence of sulphates of soda and lime in the 

 water. 



So abundant is this organic matter, that the silt of the Missis- 

 sippi water, after having been kept ten years, has the power of 

 decomposing alkaline sulphates, and forming sulphide of iron 



