Herra Thoroddsen's paper bears further evidence of having been very carelessly compiled, amongst other things, Danish measure is given for English, Askja, measuring one square geographical mile in extent and the Oddfiahraun, ' 60 square geographical miles in extent.'
Riding along the edge of the rift on the western side of the lava, the almost perpendicular and clean-cut face of its wall, from whence the tract broke away, affords an interesting glimpse at the geological history of this part of the elevated plateau in the interior of Iceland. The altitude here, by Aneroid, is 1,300 feet above sea level, and to the depth of the rift, quite 200 feet in places, we see that the plateau is built up of a succession of beds of basaltic lava, lying horizontally stratum upon stratum, with thin layers of clinker-like fragments marking the divisions between. The strata vary in thickness, but not, as far as I could see, in appearance, there being no apparent difference in the density and colour of the substrata at a depth of sixty to seventy feet-to which depth I descended into the rift-and the more superficial ones, the lava being very dense at the bottom of each stratum, becoming less so and cellular towards the surface. The lava is generally of that ashy-gray hue, peculiar to basaltic lava and basalt, but in places there are bright red and yellow patches, caused doubtlessly by the oxidation by intense heat of minerals in the lava. In places the superficial stratum is columned by vertical fractures, possibly the effects of frost; and on the whole these deposits present more the appearance of a tertiary formation of basalt, than floods of basaltic lava, which have issued subserially in comparatively recent times. These strata are covered with earthy deposits to a depth of from three to four feet only, a rusty brown loam about a foot in depth rests upon the lava, then a thin layer, about three inches in thickness, of white pumiceous earth is met with, and above this is more of the rusty brown loam, which with black volcanic sand, forms a poor soil that nourishes the scanty vegetation that clothes the moorland. The layer of pumiceous earth, I would here observe, is found spread over the whole of the north of Iceland, even on mountain ridges 2,500 feet high it is to be seen wherever the earth covering it has been washed away or otherwise removed.
Whence came the floods of basaltic lava beneath the Orcefi, and at what period of the island's history ? That they were ejected subaerially is certain, and that the air above them as they spread one over the other must have been more intensely heated than that in the interior of a furnace is equally certain, otherwise they would not have spread around so evenly or so far. It h far easier to ask these questions than to answer them, but I believe that these vast sheets of lava welled forth from a huge volcanic vent underlying Askja, whose outlet has been narrowed down by the accumulation of these deposits to ^the present dimensions of that crater. The comparatively level plateau which now forms the greater part of the interior-the more superficial strata of which are exposed by earthquake rifts in every direction-is unquestionably a more recent formation than other parts of the island, that has been upbuilded in post-tertiary times by floods of igneous rock, chiefly from a central vent. As I purpose in the concluding chapter of this work to describe the formation of Iceland, I will not further digress here than to observe that the coastal region mainly consists of semidetached flat-topped mountain masses varying little in altitude, about 2,000 feet, that appear to be fragments of a far older, more elevated, and extensive plateau than the one now forming the interior, though of very similar formation; fragments of one which, in all likelihood, was riven asunder during the disturbances of the glacial epoch, the greater portion then sinking beneath the sea. Herftubreift and several of the ice-clad Joklar in the interior, as well as the mountain masses of the coastal region, are portions of this older plateau ; and these, doubtless, at the beginning of the post-tertiary epoch formed a complex of islands very similar in appearance to the Farces of to-day, but scattered over a greater area, and have been united and formed into one large island by subsequent outbursts of igneous rock from the volcanic vent in their midst.
I am able to give the following particulars of the eruption in the Orcefi, for which I am indebted to Jon, the intelligent son of the farmer at ReylcjahMft, the nearest inhabited house to the scene of the outbreak; where, by-the-bye, I stayed in August, 1880, when I bagged four reindeer, the first of these animals, it is believed, that have fallen to an Englishman's rifle in Iceland.
Lava was first seen issuing in the Orcefi on the 18th February, forty-five days after the earthquake, but it is probable the fiery flood commenced to stream forth immediately after, no one having crossed from BeykjahliS to the eastward or vice versa during that period ; but little travelling naturally being done in Iceland during the winter. For nearly four months the lava continued to stream forth more or less freely, and then ceased to flow until the 15th August, when a smart shock of earthquake was felt, and a slight eruption of ashes and volcanic bombs took place from the rift at its northernmost end; the lava flow recommencing and continuing for several days. This eruption was witnessed by Mr. Watts, who in his work 'Across the Vatna Jokull,' very graphically describes the scene. The season of the year when the lava first burst forth, and the intermittent manner in which it issued, fully account for the chaotic confusion which the masses of the rapidly congealed earlier flows upheaved by the subsequent ones now present.
Having gratified our curiosity by exploring this, the largest lava flood that has issued in Iceland during the present century, we mounted our ponies and turned their heads in the direction of Reykfahlfo. We had not been in bed nor had a ' square' meal since leaving Husavik, consequently we were anxious to get under the hospitable roof of the unjustly maligned Petur Jonsson, where I had been made very comfortable in 1878.