106 E. W, Hilgard— Geologic Efficacy of 



formed and decomposed in presence of metallic sulphids and 

 oxids, and the solvent action so generally ensuing, to appreciate 

 the important part thej may play in this connection, especially 

 under the frequent variations of pressure likely to occur in 

 fissures, causing corresponding fluctuations in the amount and 

 nature of the gases present. The almost constant co-occur- 

 rence of calcic carbonate in gypsum deposits is exactly parallel 

 to the eifect produced by the carbonic gas passing into the 

 magma of alkaline sulphates and calcic carbonate in the funda- 

 mental experiment. The frequent paragenetic occurrence of 

 earthy and metallic carbonates and sulphates (or in the case of 

 heavy metals, of sulphids formed by the reduction of the latter) 

 suggests a possible similar origin, more readily intelligible than 

 the action of the solutions of calcic carbonate alone, vrhich is 

 commonly invoked for such transformations as that of angles- 

 ite into cerussite, or of blende into smithsonite. In both 

 these cases the alkali carbonates seem much more likely to be 

 an efficient agency, especially in view of their more decided 

 character in the excitation of electrolytic action. 



Physical Effects. — Scarcely less important than the multi- 

 farious chemical effects that may be produced by the superacid 

 alkali-carbonate solutions, are the physical effects which they 

 may and doubtless have exerted upon the consolidation and 

 lithification of sedimentary deposits containing clay. The 

 peculiar '' puddling " effect of dilute solutions of sodium car- 

 bonate upon clays, to which I first called attention in 1872, is 

 probably one of the most active agencies in converting ancient 

 soils and other clayey alluvium into the compact masses of 

 shales and hardened clays which are specially characteristic of 

 all coal-bearing formations. The potency of this action is 

 especially apparent, at this day, in the alkali lands of the arid 

 regions ; where the spots or areas subject to this action of car- 

 bonate solutions are seen to be conspicuously depressed below 

 the level of the contiguous alkali-free land, in consequence of 

 the collapse of the floccules usually existing in all soils, and 

 characteristic of all recent alluvial deposits, as a result of the 

 currents and counter-currents under whose influence they have 

 been formed. The soil in such areas often becomes so far con- 

 solidated as to resist all efforts at tillage or pulverization, so 

 that even blasting with dynamite will sometimes only throw 

 up large chunks of consolidated soil. The clay hardpan thus 

 formed in the depths even of lands not so heavily impregnated 

 is of a similar nature, and forms one of the most formidable 

 obstacles to the reclamation of such lands. It can in fact be 

 overcome only by the chemical decomposition or transforma- 

 tion of the alkali carbonate into a salt of neutral reaction, such 

 as the sulphate ; most readily accomplished by the agency of 



