Professor Johnstrup states that the lava on the eastern side of Askja has a declination towards the gap in the north-east of 300 Danish feet in a stretch of 12,000 = to 1 in 40.
Now, as at the time of my former visit, the floor of Askja, notwithstanding it lies 3,700 to 3,800 feet above sea-level, was bare of snow. This, one would imagine, must be owing to internal heat, as twenty-five miles to the southward we have the glacier-covered region known as the Vatna Jokull, with a mean altitude of less than 5,000 feet; and in the north-west of Iceland, at an altitude of less than 3,000 feet, the icy-wastes of the Glamu and Drdnga Joklar, which tracts cannot possibly be more favourably formed for glacial deposit than Askja's amphitheatre. Professor Johnstrup says that in Iceland the snow line can be set at about 2,500 feet .(Danish). The whole surface of Askja, save in the south-east where there is an extensive tract covered with pumice erupted in 1875, and a tepid lake five miles in circumference, is a chaos of rugged lava-floods that have issued here at different periods. From those on the left, looking south across the crater, for an area exceeding a square mile, ascend innumerable small jets of steam; and there is a solfatara of small extent between this tract and the mountain-wall on the north. These do not mark a site of disturbance during the 1875 eruptions, however, the situation of the rifts and vents then opened being clearly indicated by the enormous volumes of steam that belch forth on the further side of the crater, close under its encircling mountain-wall in the south-east.
The five miles of rugged lava and pumice between us and the vents opened in 1875, I knew only too well, would have to be crossed on foot; therefore, after partaking of some food, H., Arni, and I started, leaving Einar in charge of the ponies. In 1878 we hobbled our animals, and left them unattended at the foot of the pass, to discover upon our return that two of them had broken their hobbles and made tracks for the Siiftrd, and that the third was anchored fast half-way up the ashy slope by a large stone to which, being a very valuable animal, he was fastened by a lariat for greater security: consequently, Einar, I, and another man-the Deputy Sheriff, by-the-bye, and a first-class fellow-after being a day and night out of bed had to return across the Oddftahraun on foot, the remaining pony being laden with the saddles of the others. How ever I accomplished the journey I do not know to this day, for I was delirious when I reached Einar's roof.
The crossing of the lava-covered floor of Askja is most fatiguing work; it has taken me, a young and active man, each time that I have crossed, four hours to proceed as many miles, most of the way by the aid of my hands protected by thick woollen mittens that they might not be cut by the lava. I may also observe for the benefit of anyone who in the future may visit this volcano, that the crossing of Askja utterly ruins the pair of boots worn, the sharp edges of the lava cutting through the sides of the uppers like knives, so that an old pair with good but not thick soles should be taken, as their destruction would not prove so great a loss as that of a newer pair.
The superficial lava in Askja is pronounced by Professor Johnstrup to be basaltic; and it has evidently issued at different periods, some tracts being covered with a lichen growth, while others are as rugged, black, and new-looking as those lava-floods near Myvatn which issued but a century-and-a-half ago. I saw no lava anywhere so new-looking that it could possibly have been erupted as recently as 1875.
When one has succeeded, after many rests and tumbles, and much scrambling, in approaching within a mile or so of the bursts of steam, he is able to walk upright, and proceed more quickly; the lava being buried beneath a covering of pumice which gradually increases in depth as the vents whence it was erupted are approached. The pumice is of three colours: black, light-silvery-gray, and golden-brown; the last-named very fibrous and presenting the appearance of masses of the outer-husks of gigantic cocoa-nuts, and when blocks of the two last-named were broken, the newly exposed surfaces respectively glistened like silver and gold. This substance must have been much harder and tougher when ejected than now, for the blocks, some of which were three to four cubic-feet in bulk, when lifted breast high and let fall, not cast to the ground, broke into fragments by their own weight. Yet the blocks experimented upon must have been cast to and fallen from a considerable height, as they lay half-a-mile from the nearest of the newly-formed craters. The pumice, owing to the expansion by frost of the snow and rain which find their way into its pores, is fast degrading into a pumiceous sand. Amidst the pumice are a number of immense blocks of obsidian and pitchstone, some of which must be several tons in weight. These had evidently been erupted at the same time as the pumice, or subsequently.
The apex of the slope thus formed is a cone-shaped crater, about 200 feet in height above the superficial lava in Askja. When I was here in 1878, tremendous blasts of steam were belching forth almost continuously with perfectly deafening roars, a hundred times louder than those concomitant to the blowing off of steam from the boilers of the largest of Transatlantic steamships; but now, to my astonishment, all was still, and upon climbing to the summit I found that in the crater, at the depth of about 150 feet, was a placid pool of apparently cold water, with a number of small stufa emitting inconsiderable jets of steam within ten feet of its surface. The diameter of the crater at its mouth is at least 500 feet, and the interior is an inverted cone-shaped hollow, which decreases to one-third of that diameter at the level of the water. In 1878, there being no water in the crater, I could see that at about a depth of 200 feet a flat shelf, presumably the lava-strata in Askja, twenty feet in width, encircled a well-like opening in the centre, through which the steam belched forth. The walls of the crater are formed entirely of pumiceous sand and a clayey loam. I saw no traces of scoriae, or lava-like slag. This crater is beyond a doubt ' the shaft like the mouth of a large coal-pit,' mentioned by Mr. Watts, though it is not (situated in the N.N.E. corner,' but in the south-eastern part of Askja, there being no other vent here that will answer Mr. Watts' description. This crater in its present condition is a twin-brother to the large one on the western slope of the volcano Krafla, known as Helvita stcerra (Greater Hell).