CHEMISTRY OF THE EARTH. 193 



favored the accumulation of the great beds of gypsum and magnesiau 

 limestones which generally accompany the salt deposits of past geologi- 

 cal periods. The hydrated magnesian carbonate, whether the con- 

 comitant of gypsum, as in this case, or of chloride of sodium, as in the 

 former reaction, unites chemicallj" with the associated carbonate of lime, 

 and gives rise to dolomite or magnesiau limestone.* 



§ 21. The action of carbonated alkaline waters on the salts of the sea 

 under ordinary conditions thus gives rise to carbonate of lime, and it is 

 oidy ui^der peculiar circumstances that magnesian carbonate is sep- 

 arated. The case is, however, changed Avith silicated alkaline waters 

 coming from deep seated silicated rocks, which undergo a decomposi- 

 tion without the intervention of the atmospheric air, and hold dissolved 

 silicates of alkalies and of lime. These reactiDg ou the magnesian salts 

 dissolved in sea-water give rise to magnesian silicates, which are very 

 insoluble. Hence we frequently find deposits of magnesian silicates 

 in sediments, while silicates of lime are comparatively rare. In the 

 solubility of bicarbonate of magnesia and the insolubility of the cor- 

 responding lime salt, and in the insolubility of magnesian silicate and 

 the solubility of silicate of lime, we find a simple explanation of the geo- 

 logical relation of calcareous and magnesian silicates and carbonates.f 



§ 22. The relations of the alkalies, potash and soda, require some 

 consideration in this connection. The silico-aluminous componiids of 

 potash possess a much greater degree of stability than those of soda. 

 This is exemplified in the case of rocks which contain, side by side, 

 orthoclase and albite, or oligoclase, when it is often found that the soda- 

 feldspar has undergone decomposition from a loss of a portion of its 

 alkali and partial disintegration, while the orthoclase or potash-feldspar 

 remains uuchangcd. It is well known that waters holding large portions 

 of potash salts in solution, exchange the potash for soda when filtered 

 through a stratum of earth in which the amount of potash is, neverthe- 

 less, as great or gTcater than the soda ; and we find that in natural spring- 

 waters, which often contain considerable amounts of alkaline carbonates, 

 the proportion of potash to the soda is as small as in the ocean. Sur- 

 face-waters bearing the unfiltered wash of the land carry considerable 

 portions of potash to the sea, but it is constantly removed, partly, at 

 least, by the agency of fucoids, which, as Forchammer has shown, like 

 land-plants, take up large amounts of potash, and subsequently, by 

 their decay in contact with the argillaceous mud, restore the alkali in an 

 insoluble form to the earth. The formation of glaucouite, a peculiar 

 silicate rich in potash, which has been going on in the bottom of the sea 

 from a very early period to the present time, has also been constantly 

 withdrawing the potash from the ocean, so that soda is still the predomi- 

 nant base in its waters. 



§ 23. The changes of silicated rocks under the influence of water, 

 carbonic acid, and the products of decaying organic matter, present 

 several points of interest. The chemical decomposition of feldspars con- 

 sists in the removal of their protoxide bases, alkalies, and lime, together 

 with a portion of silica, leaving as a final result a hydrous silicate of alu- 

 mina or clay. This change is favored by mechanical division, and Dau- 

 bree has shown that by the prolonged attrition of the particles of granite 

 under water, the softer and cleavable feldspar is, in great part, reduced 

 to an impalpable powder, while the uncleavable quartz forms rounded 

 grains of sand, the water at the same time dissolving from the feldspar a 



* T. S. Huut, Ou the Salts of Lime and Magnesia. American Journal of Science, [2,] 

 •slii, 49. 



^ t T. S. Hunt, American Journal of Science, [2,] xl, 49. 

 13 s 



