7Wnnp of Hot Water, Air Volcanoes md Cold Springs. S09 
bles earthy slag. It is formed by the hot-springs of Leyraa, 
and also by the waters of many other hot-springs in Iceland. 
Besides these mineral productions of hot-springs, the Danish 
naturalists enumerate the following, which are of a looser texture: 
1 . Bolus violacea. This violet-coloured bole or clay effer- 
vesces with acids, and is found in great quantity around active 
hot-springs, and in extensive districts where such springs for- 
merly existed. 
S. Bolus thermarum ccerulea. This blue bole contains im- 
bedded crystals and grains of iron-pyrites, occasionally sulphur, 
and has a sourish taste. Some of the eminences formed of it are 
still hot, so that if we dig three or four feet deep into the bole, 
the heat is so great that we cannot hold our hand in it. Whole 
hills are formed of this bole, which, in such cases, is frequently 
disposed in strata. 
3. Bolus hriinnea. Tripoli of Iceland. This rock is hard, 
and sharp to the feel, and has many of the characters of Tripo- 
li. It appears to be a thermal bole* or clay, impregnated with 
silica. The water of hot springs flowing over such a rock, 
might give rise to veins and masses of pearl-sinter, hyalite, 
opal and obsidian. 
Such are the observations of Olafsen and Povelsen, which ap- 
pear to shew the formation of columnar and massive rocks by 
hot-springs, and also of obsidian, hyalite, pearl-sinter and opal. 
Dr Henderson, in his Travels in Iceland, alludes to the forma- 
tion of rocks by hot-springs ; but Sir G. Mackenzie, in his work, 
does not refer any of the rocks he met with in Southern 
Iceland to the thermal class, Menge, an active naturalist, was 
aware of the interesting and important nature of these thermal 
formations. He observed thermal rocks having many of the 
characters of basalt, porphyry, wacke, amygdaloid, tufla and 
obsidian. These he met with in districts '^^here formerly hot- 
springs occurred ; and observed them still forming around many 
of the magnificent hot-springs at present in a state of activity. 
These thermal trap-rocks, viz. basalt, wacke, amygdaloid with 
calcareous-spar, porphyry, &c. are brought from the interior of the 
earth, by the water of hot-springs, partly in a state of solution, 
partly in a state of mud, and are deposited over flat of hilly 
tracts of country, when they gradually harden j sometimes cry- 
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