' The earthquake then,
As a beast in pain,
In his burning den,
Snapt his chain: Till bound at last, like a snake he curled,
And formed the mountains of our world.'
IN this chapter I purpose bringing this monograph to a conclusion with a few observations upon the genesis and probable history of Askja, and likewise upon the part played by that volcano in the formation of Iceland; but before doing so, I will, now that my reader has figuratively accompanied me over both the scenes of eruption in 1875, give my reasons for asserting so positively that the earthquake which opened the rifts in the Myvatns Orcefi was caused by an explosion in Askja, and that the lava came thence through a subterraneous channel. Fire, smoke, and steam were seen ascending from Askja immediately after the earthquake on the memorable 4th January, and on the night of the 9th the column of fire was so high and brilliant, that according to a letter written by a resident of the capital-which is quite a hundred miles distant: ' all agreed that it was some neighbouring farm burning, with haystacks. The fire shot up like lightning, . . . when daylight dawned, and we could discern the mountains, we observed a thick and heavy column of vapour or steam far in the background beyond all the mountains visible. . . Morning and night this grand display was visible during the 9th, 10th, 11th, and 12th, and during the day the column of steam and smoke stood high in the sky.' During the whole time that lava was flowing forth in the Myvatn's Orcpfi smoke and steam were seen ascending from Askja; and prior to the outburst of lava being discovered, explosion after explosion took place there with such violence and frequency that Jon of Viftikcer, as before related, ventured to cross the great central lava-desert, in the coldest month of an Icelandic winter, to see what was taking place. Moreover, while no lava was erupted from Askja itself, the quantity of pumice there ejected was prodigious, and proportionate to the magnitude of the lava-flood that issued in the Orceji, where only an infinitesimal quantity of pumice and ash was thrown out; and these facts, taken conjointly, are almost irrefragable proof that the lava came thence, for pumice is asserted by mineralogists to be the scum which forms on the surface of lava when in a molten state.
The beds of basaltic and doleritic lavas underlying Askja, bared in the face of the cliffs bordering the tepid lake on the north, are conclusive evidence that a huge cauldron-shaped hollow formerly existed in the heart of the chief of the Dyngjufjoll, which has been filled up to its present level by the lava-floods that have been deposited therein one above the other. It is reasonable to believe that in the course of time these vast deposits first narrowed and then entirely blocked up the vent or vents whence the lava issued, so that in 1875 it required less force to upheave, at a lower level by some 2,400 feet, the rocky roof of an old channel * running under the Orsef, connected with the volcanic outlet amidst the Dyngjufjoll, than to force a vent through the lava deposits in AsJcja itself. To judge from what occurred at both places, it is probable that the force required to burst asunder the strata forming the roof of the channel in the Oriefi, and that necessary to force an outlet in the crater, were nearly equal, and that at the same moment the rocky roof of the channel lifted in the desert, the huge oval mass that has sunk in Askja was also forced from its bed. Naturally, vents having been formed in two places, the heavier molten matter would flow from those at the lower level, while the steam and gases would make their escape, carrying with them the lighter substances, such as pumice and ashes, from the vents at the greater altitude. The latter, moreover, would act in a manner as safety valves, and the molten lava, not being subjected to any great pressure from confined steam or gases, by its own gravity would well gently forth through the newly-formed vents at the lower level, instead of being forcibly hurled far and wide by the enormous pressure of confined steam and gases generated above it, as most certainly would have happened if the ' safety valves ' in Askja had not existed.
* The course of this channel is, in all likelihood, marked by the depression in the Myvatns Orcefi, running northward from the 1875 lava-flood, bordered on the east by an old rift known as the Sveinagjd, as at a spot where a line continued from it in a north-north-easterly direction would intersect the bed of the Jokulsd, the river bordering the desert on the east, there is a group of small new-looking crater-cones, and the surface of the earth has been greatly disturbed by recent earthquakes. The Sveinagjd runs some distance southward beyond the northern end of the 1875 lava-bed; and the subsidence there took place between this older rift and a newly-formed one.
Fortunately for the inhabitants of Iceland this is exactly what occurred, as we have seen.
The shock of earthquake on the 15th August, and the slight eruption of ashes and volcanic bombs in the Oreeji from the crater-cones at the northern end of the lava bed are, I think, easily accounted for. It is extremely likely that a quantity of water had been gradually collecting in the hollow in Askja, now the bed of the lake, and that at this time it suddenly found its way through a newly-opened fissure into the abyss beneath (that one exists the subsidence conclusively proves), in which the molten lava had then fallen so low that its surface lay lower than the level of the roof of the subterraneous channel connecting Askja with the scene of eruption in the Oreeji, that the steam generated caused the shock of earthquake, and forced out the pumice and other substances forming a scum on the lava in the channel. The fact that lava had ceased to issue for some time previous to this shock of earthquake confirms the view taken, that the channel was only partly filled with lava, so that a scum could form upon that lying therein ; and from what was taking place in Askja a month earlier, when Mr. Watts was there, it is not very speculative to imagine that a fissure was newly opened, and that through it the water from the snow, rapidly melting under the summer sun, found its way into the heated abyss.