102 E. W. Hilgard — Geologic Efficacy of 



but when, instead of the alkali sulphates, the chlorid solutions 

 are employed, the result is qualitatively the same ; the highly 

 soluble CaCl^ coexisting in solution with the sodic or potas- 

 sic hydro-carbonate. 



Now when we consider the almost universal co-occurrence 

 of the three factors, viz. calcium and magnesium carbonates, 

 alkali chlorides and sulphates, and free carbonic acid in nature, 

 both at the surface and in the geological strata, it is easily seen 

 that this simple process of formation of the alkali carbonates 

 must be of vastly more frequent occurrence than that which 

 presupposes the existence of soluble alkali silicates. The fact 

 that, as stated above, the evaporation residues of such well and 

 spring waters nearly always show an alkaline reaction, is thus 

 explained very simply, upon the easily verifiable supposition 

 that they contain, not the normal salts, but in part at least the 

 super-carbonates, whose presence is compatible with that of 

 soluble earth salts.^ 



When solutions prepared artificially as above described, and 

 filtered, are then evaporated hot, the reaction goes back to a 

 considerable extent, earth carbonate and alkali sulphate being 

 again formed. £ut no amount of boiling will entirely destroy 

 the alkaline reaction : as might be expected from the well 

 known fact that in order to obtain the normal carbonate with 

 certainty, it is necessary to use a red heat, and that all alkali 

 carbonates exposed to atmospheric air absorb a small excess of 

 carbonic acid. When, on the other hand, solutions thus pre- 

 pared are allowed to evaporate spontaneously, at the ordinary 

 temperature, the going-back occurs to a very much less extent, 

 and in the case of soda especially the crystalline residue con- 

 sists largely of " sesqui-carbonate." The curves given on p. 103 

 show at a glance the transformation of the sulphates of sodium 

 and potassium into carbonates, under the influence of calcic 

 carbonate and excess of carbonic acid in progressively stronger 

 solutions. Corresponding experiments made with magnesic, 

 instead of calcic, carbonate promptly showed that the same 

 reaction occurs with at least equal facility. This renders it 

 evident that the same must be true of many other earthy and 

 metallic carbonates, but only the barium salt has been actually 

 tried thus far, but with the expected result. As our systematic 

 experiments were carried out with calcium carbonate only, the 



* A notable case of the direct formation of alkali carbonates in rock decompo- 

 sition occurs in the northwest region of the Pacific Coast, where waters contain- 

 ing alkaline carbonates almost alone, with only a trifling amount of silica, issue 

 directly from the crevices of the bare basaltic rock, which of course contains 

 little or no sulphur compounds ; but the sulphates become abundant wherever this 

 rock is associated with the later sedimentary deposits. Here, if anywhere, one 

 would have expected to find alkali silicates as a direct result of rock decomposi- 

 tion by carbonic acid. 



