258 Natural History of Voleanos and Earthquakes. 
riations. Channels may become stopped by the incrusting prop- 
erty of the water. During the frequent shocks which accom- 
pany the greater eruptions, some cavities may fall in, and be 
choked up, and new ones formed. The greatest changes, how- 
ever, 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, disap- 
peared, and at present only steam is evolved from its mouth, 
while the Strokr, which before this was but an inconsiderable 
spring, 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 Strokr have no connection whatever with those 
of the Great Geyser. During the long eruption of the former, 
the latter remained 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 the 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 
vapor and sulphureted hydrogen gas ;) that these springs ate 
supplied from the meteoric waters of the neighboring hills, an 
that, being originally cold, they are indebted for their high tem- 
perature solely to the hot vapors which they receive from below. 
The hot springs in Iceland seem, therefore, to be produced in the 
manner described at page 253. 
Lastly, If the permanent obstruction of the lava and the water 
channels has taken place, of course no hot springs can exist, OF 
at least they can only flow during the cooling of the lava last 
ejected and solidified. This seems to have been the case in the 
volcanic district of the Siebengebirge, the Laacher See, and the 
Bifel, as in these places no hot springs, with the exception of 
the baths of Bertrich, are to be met with; although in the two 
