220 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



layer of double chlorides of potassium and magnesium. These 

 salts, in spite of their high deliquescence, have been preserved 

 from denudation in an exceptional degree owing to the presence 

 of a thick protective surface- man tie of clay. 



The subject was treated by E. Reichhardt (1866), and still 

 more successfully by F. Bischof (1875), upon the recognised 

 principles of desiccation. 



Ochsenius in 1875 set forth the nature of the conditions 

 under which the Stassfurt succession might have been formed 

 in nature. He supposes a bay or a sea-basin connected with 

 the main ocean only by a narrow channel, which was periodi- 

 cally closed by crust-movements, or by the accumulation of 

 sandbanks or submarine bars which could be surmounted only 

 at the highest tides. During the period of closure, wherever 

 the evaporation exceeded the inflow of fresh water, a concen- 

 tration of the salt water would take place, and gypsum, an- 

 hydrite, and salt would be thrown down. If a permanent 

 isolation were finally effected, and desiccation brought about in 

 this natural salt-pan, it followed that the salt of the mother- 

 liquor must separate out completely in accordance with the 

 order of their solubility. 



C. Geological Effects of Ice. The importance of ice as a geo- 

 logical agent was much later in being recognised than that of 

 water, and this is readily explicable from the more limited 

 occurrence of ice and the less striking character of its action. 

 Moreover, the regions where ice displays its grandest effects 

 were still avoided in the eighteenth century, and were only 

 familiar to a few bold explorers. The river and lake ice of the 

 continents, and the ocean ice of the Polar districts have little 

 interest for geologists, since they cannot help much in eluci- 

 dating the work of ice in the past epochs of the earth's history. 

 Greater interest attaches to the glaciers of the- mountain- 

 systems and the inland ice-sheets of the Polar continental 

 areas. 



Glaciers are mentioned for the first time in literature as 

 a subject of scientific investigation in Scheuchzer's Reisebe- 

 schreibung der Schweizer A/pen. The indefatigable and 

 learned scientist records the few observations of Simler and 

 Hottinger on the origin and movement of glaciers, and after 

 a careful description of several glaciers visited by himself, 

 he explains the movement as a result of the infiltration and 



