CHAPTER IX. 



ORIGINAL STRUCTURES AND LITHOGENESIS OF THE TRUE 

 AQUEOUS OR HYDROGENIC ROCKS. 



True aqueous or hydrogenic deposits, i. e., precipitates from 

 solution in water, may be grouped under a number of divisions, ac- 

 cording to the mode of origin. The principal groups are : 



1. Marine, or oceanic (halmyrogenic, halogenic,* or thalasso- 

 genic f). 



2. Lacustrine (limnogenic $), including those of lakes, ponds, 

 marshes, salinas, and playa lakes. 



3. Fluvial (potamogenic §). 



4. Terrestrial, including those of springs, both cold and hot, of 

 geysers, the deposits in caverns, mineral veins, etc. They comprise 

 deposits of both vadose and magmatic waters. Under this head 

 must also be placed the salitrales of Patagonia and other regions. 



Chemical deposits of the open sea when not alteration products 

 are practically limited to carbonates of lime and of magnesia, though 

 these are rare in the modern ocean. In enclosed mediterraneans or 

 other cut-ofifs from the ocean, deposits of calcium carbonate are 

 formed, while gypsum and salt may be precipitated on evaporation 

 of the water. Complete evaporation may result in the deposition of 

 the rarer salts, especially those of potash. 



In lakes chemical deposits are chiefly confined to the carbonates 

 of lime, which may be extremely abundant. Complete evaporation, 

 however, may result in the deposition of a variety of salts, including 

 chlorides, nitrates, borates, sulphates, carbonates, etc. Chemical de- 

 posits of fluvial origin are chiefly limited to the carbonates of lime, 

 though deposits of iron oxides and carbonates may also be formed. 

 Finally, the terrestrial deposits of this type include carbonate of 

 lime and silica, as well as a large variety of additional mineral 



* dXs = salt. 



f daXacrcra = the sea. 



^Xl/xvtj = a marsh, lake, pond. 



§ TTOTafxSs = river. 



329 



