411 



Naples gives off steam and sulphur-compounds which decompose the 

 trachytic rock and lead to the formation of gypsum and alum. Pyrites 

 occurs as a secondary constituent of many volcanic rocks. It may be 

 formed by the action of sulphuretted hydrogen on the iron-compounds 

 present in the original rock. 



Thermal waters charged with various substances in solution are 

 common in volcanic districts. These are potent agents in rock-metamor- 

 phosis. The water of the Great Geyser of Iceland contains 2'448 cubic 

 centimetres of sulphuretted hydrogen per litre according to DES CLOISEAUX, 

 and leaves on evaporation 1225 milligrammes of solid matter per litre 

 according to M. DAMOUR. The composition of the water of the Great 

 Geyser has been studied by SANDBERGER and DAMOUR, with the following 

 results : One thousand parts of water contain (A) according to 

 SANDBERGER and (B) according to DAMOUR^ : 



A. B. 



Si0 2 ... 0-5097 ... 0-5190 



Na,,CO, ... 0-1939 ... 0-2567 



Na,S0 4 ... 0-1070 ... 0-1342 



K 2 S0 4 ... 0-0475 ... 0-0180 



MgS0 4 0-0042 ... 0-0091 



NaCl ... 0-2521 ... 0-2379 



It seems probable, as DAMOUR has pointed out, that the silica was 

 originally in combination with the bases, soda and potash, in the form of 

 alkaline silicates, and that these silicates have been derived from the 

 subterranean decomposition of felspathic rocks. If so, it follows that a 

 process of subterranean kaolinization must be going on. At present, 

 however, we are not concerned with the subterranean effects produced by 

 percolating waters, but with the changes which thermal waters charged 

 with silica and other substances may produce on igneous rocks at or 

 near the surface. Assuming that the geyser-water in the first instance 

 contained alkaline silicates then we see that these silicates are decomposed 

 by the action of hydrochloric, carbonic and sulphuric acids the silica is 

 liberated and carbonates, sulphates and chlorides of the alkalies are 

 formed. The water of the Great Geyser deposits a siliceous sinter 

 (geyserite), having the following composition according to FORCHAMMER : 



SiO,, ... 84-43 



Al,6 3 - 3-07 



Fc 2 3 ... 1-91 



CaO ... 0-70 



MgO ... 1-06 



K 2 0(Na 2 0)... "92 



H 2 0&C ... 7-88 



99-97 



Now if water, having the composition of that of the Great Geyser, 

 percolates through porous rocks it will deposit a portion of its silica in 



(1) Quoted from ROTH'S Allgemeine und Chemische Geologie, p. 592. 



