288 THE GEYSERS OF MONTANA 
again toward the colder surface, and is there condensed into hot 
water. The over-heated steam, however, and the gases decompose 
the rock beneath, dissolve certain ingredients, and deposit them 
on the surface. According to Bunsen’s ingenious observations, a - 
where rising sulphur vapor comes into contact with glowing 
masses of rock. Wherever vapors of sulphurous acid are con- 
stantly formed, there acid springs, or solfataras, arise. Incrusta- 
tions of alum are very common in such places, arising from the 
action of sulphuric acid on the alumina and alkali of the lavas; 
another product of the decomposition of the lavas is gypsum, or 
sulphate of lime, the residuum being a more or less ferruginous 
fumarole clay, the material of the mudpools. To the sulphurous 
; TER 
Professor Bunsen, rejecting the antiquated theory of Makenzie, 
based upon the existence of subterraneous chambers, from whic 
of a frame of silicious deposits, with a deep, flue-shaped tube, and 
pon den development of larger masses of steam from 
_ observed in these springs. 
_ The rocks, from which the silicious hot-springs of New Zea- 
-~ land derive their silica, are rhyolites, and rhyolithic tufas, contain- 
ing seven more per cent. of silica; while we know that in 
A 
a: 
