PEALE.] CARBONATES, SULPHUR, ETC., IN THERMAL WATERS. 397 



deposit of Pealite, wliicli ^v^as considerably fractured. On digging into 

 it steam was noticed coming through the cracks, and on these surfaces 

 the deposit was soft and could be cut with the knife like soap. This 

 softening of the silica (the analysis shows it to be 75 per cent, of silica) 

 was due to the effect of the steam. 



During the eruption of Vesuvius in October, 1821, Monticelli and 

 Covelli noted that fragments of lava when no longer hot remained sep- 

 arate, but when very hot or traversed by hot vapors they united so firmly 

 that heavy blows of the hammer only could break them.* 



SOUKCE OF THE CARBONATES. 



When the thermal waters are found in connection with limestones we 

 find that they usually contain large percentages of calcium carbonate, 

 which deposits in the form of calcareous tufa, the most common form of 

 sediment found in connection with springs. All the springs in the Yel- 

 lowstone i^ational Park found with such deposits are found to have some 

 connection with limestones. Carbonated waters coming in contact with 

 silicates in granite, basalt, «&c., subject them to a decomposition, and, 

 as a result, the carbonates are formed. 



SULPHUR AND SUXPHURETED HYDROGEN. 



Sulphur is a prominent constituentof many springs, and sulphureted 

 hydrogen is, next to carbonic acid, the gas most frequently found in 

 springs. Sulphur is supjjosed to be due to the decomposition of sul- 

 j)hides by the action of organic matter. In this connection it is inter- 

 esting to note that the localities of the Yellowstone Park in which sul- 

 phur is characteristic are those where there are lake beds, that is, where 

 the rocks were deposited at the bottom of lakes. This is especially the 

 case in the Haydeu's Valley springs. In connection with these sediments 

 there is doubtless a considerable proportion of organic matter from the 

 vegetation that once existed in the bottom of the old lake. 



M. Eugene Planchudf believes the presence of sulphur is due to the 

 reducing action of vegetable matter, basing his views upon the exami- 

 nation of the long, cylindrical sulphur threads found in the neighbor- 

 hood of sulphur springs. It may be partly due, also, to the decomposi- 

 tion of sulphides by extremely hot water. In the Gi()bon Geyser Basin 

 we are led to suspect that the hot water must somewhere come in con- 

 tact with sulphide of iron, which it decomposes. Iron and sulphur are 

 abundantly present there. 



ALUMINA. 



As we have noted in a preceding portion of the report, the alumina 

 of most of the springs is due to the fact of the water i)assing through 

 beds of clay. Where the water contains sulphur we frequently have 

 sulphides formed, as in the springs at Crater Hills in Haydeu's Vallej\ 



* Cited by Bischof, Internal Heat of the Globe lloc. cit,'], p. 216. 

 t Vol. 41 Choniical News, p. '236. 



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