696 DYNAMICAL GEOLOGY. 



produced when alumina is present. Different sulphates of iron, or 

 vitriols, and some related products are other results. 



6. By Deoxydation. — Organic matters, owing to their tendency to 

 oxydation, may take oxygen from sesquioxyds, and make protoxyds of 

 them ; so that carbonic acid, if at hand, can combine with the iron,. and 

 form carbonate or bicarbonate ; and organic acids, as Hunt has urged, 

 may form soluble organic compounds. In the decomposition of a 

 rock containing feldspars, in which iron is present, the clay, when first 

 made, is usually colored ; but after the bed of clay has thickened 

 to a few inches or feet, it is often found that the oxyd of iron has all 

 been washed out, leaving it nearly or quite white. This is accom- 

 plished by the process of deoxydation just mentioned. By means of 

 it also, large beds of carbonate of iron have sometimes been formed. 



In a similar way, sulphates have been reduced to sulphids. In the 

 black marsh-mud deposit of the Quaternary of Louisiana, there is 

 some pyrite, derived through the deoxydizing process of organic 

 matters. (Hilgard.) 



7. Through the Evaporation of Sea-water, and attendant Chemical 

 Changes. — The ocean is a mineral spring that dates from the period 

 in the earth's history when the vapors first settled on the cooling crust. 

 All the materials that were at all soluble, and that the conflict of hot 

 rocks and hot waters could have then made, were at first present in 

 it. An excess of phosphates and of carbonate of lime continued to 

 characterize it after the Paleozoic era had begun, as is learned from 

 the abundance of Lingulce and other phosphatic shells (pages 59, 593), 

 and the profusion of other shells, and of corals. At present, and since 

 Paleozoic time began, the only chemical deposits abundantly made 

 from the waters, in confined basins where evaporation was possible, 

 appear to have been gypsum and common salt. But, with these, some 

 magnesian minerals have been produced, and also some deposits of 

 borates. Salt deposits are now in progress, in confined salt-water 

 basins, along-side of low seashores. 



The production of magnesian carbonate of lime (dolomite) has been attributed to the 

 reaction of the magnesian salts of the ocean's waters, in evaporating basins, on the cal- 

 careous material of the bottom. The magnesia can have come only very sparingly from 

 corals, shells, or other calcareous relics, animal or vegetable, and must, therefore, 

 have been introduced from outside. As the dolomites are of all ages, include the 

 majority of the earth's limestones, and have often a wide continental extent, no mag- 

 nesian mineral springs can be adequate for their production, excepting the great ocean 

 itself. The chemistry of the process is not yet fully understood. 



In the preceding pages on water as a chemical agent, only the 

 more prominent and obvious of the results have been considered. All 

 the ingredients of mineral springs have done work, in the way both of 

 destruction and of construction. A full discussion of the effects be- 

 longs only to a treatise on chemical geology. 



