83 



which ahnost entirely constituted the rocks of Iceland, and indeed all 

 the Icelandic rocks that he had met with had been either igneous in 

 their orii^nn or derivatives from igneous rocks. They might divide the 

 productions of the Icelandic volcanoes into two classes. First, those 

 in whose formation water had played a prominent part— these they 

 might call the agglomerates or tuffs ; and secondly those which were 

 entirely of igneous origin, in other words, the lavas. The first of these, 

 that is the agglomerates, were formed by the bursting of mountain 

 lakes and the sudden inelting of the frozen covering of snow-clad 

 mountains during periods of volcanic eruptions, which caused an 

 overwhelming flood of heated water to rush down the mountain's side. 

 This, licking up the sands, the fragmentary debris, and whatever 

 accumulation it might meet with in its course, precipitated itself in a 

 terribleandheterogeneous avalanche upon the plains below, forming hills 

 of steamingagglomerate,aadfillingup whole valleys withaseethingpaste. 

 But the mountain streams and atmospheric elements collected together 

 through the long periods precisely the same constituents as formed the 

 agglomerate of more terrible manufacture. The mountain stream 

 brought down grain by grain the sand, the constituents of disinte- 

 grated rocks, and piece by piece the fragments that had been split off 

 by the action of the frost from overhanging peaks. The mountain 

 stream worked slowly, but as surely as the dread avalanche of fire and 

 water. Thus even in the very home of convulsion we found the 

 gentle and constant forces of Nature vieing with the terrible, and 

 showing that the peaceful and sustained efforts of Nature did as 

 much towards altering the face of the earth as the most fearful con- 

 vulsions ; but as a rule, their effect differed in this way, the peaceful 

 forces of Nature gave us our beautiful scenery and smiling landscapes, 

 while the terrible gives us the awful magnificence and grandeur of 

 desolation so pecular to volcanic scenery. 



The second class of Icelandic rocks was the lavas. These occurred 

 either as stony streams that had flowed from volcanoes, or as pummice 

 which had been hurled high into the air, and fallen in a destructive 

 shower of vesicular cinders. There was another class of lavas, namely, 

 the glassy variety, or obsidians. These, on account of the extra 

 amount of silica they contained, and in all probability the different 

 circumstances attending their cooling, formed a volcanic glass, 

 occuring in glassy or semi-glassy masses, also as a beautiful vitreous 

 pummice, such as that which was ejected from the volcano of Oskja-gj^ 



