Volcanos and Earthquakes, SSf, 



variation s. Channels may become stopped by the incrusting 

 property of the water. During the frequent shocks which ac- 

 company the greater eruptions, some cavities may fall in, and 

 be choked up, and new ones formed. The greatest changes, 

 liowever,, are caused by the earthquakes, which from time to 

 time visit the island. Thus, during the earthquake of 1789, 

 the most important spring in the country, next to the Geyser, 

 disappeared, and at present only steam is evolved from its mouth, 

 while the Sirokr, which before this was but an inconsiderable 

 sjoring, increased to such an extent, that it is now considered 

 to rival the Geyser in importance. It may be observed, that 

 the eruptions of the Stroikr have no connection whatever with 

 those of the Great Geyser. During the long eruption of the 

 former the latter remained quite quiet, and vice versa. In 

 general, each of these numerous hot springs, which are here 

 crowded together in a very small compass, seems to be totally 

 independent of each other. This might also be inferred from 

 the striking difference in their levels. 



It seems probable from the situation of the celebrated hot- 

 springs of Iceland (of which more than fifty may be counted in 

 a space of a few acres, at the foot of a rock about 300 feet high, 

 which leans against a chain of higher rocks) ; from the numerous 

 fissures in these rocks, which are composed of alternate layers 

 of tuffas, of slag-streams, and slag-conglomerates, as well as 

 from the fact, that tlie springs are confined exclusively to the 

 lower region, which extends along the foot of the hill, whilst 

 on its sides and summit are found only gaseous exhalations 

 (aqueous vapour and sulphureted hydrogen gas) ; that these 

 springs are supplied from the meteoric waters of the neighbour- 

 ing hills, and that, being originally cold, they are indebted for 

 their high temperature solely to the hot vapours which they 

 .ceive from below. The hot-springs in Iceland seem, therc- 

 xore, to be produced in the manner described at page 50. 



Lastly, If the permanent obstruction of the lava and the water 

 channels has taken place, of course no hot-springs can exist, or 

 at least they can only flow during the cooling of the lava last 

 t jected and solidified. This seems to have been the case in the 

 volcanic district of the Siehengehirge, the Laacher See, and the 

 Eifel, as in these places no hot-springs, with the exception of 



