* It is said by the people living around Myvatn, who scour the Orcrfi every autumn in search of strayed sheep, that a similar subsidence is to be seen at the southern end of the lava-bed. I cannot say whether this is true or not, having only ridden southward along the lava for a distance of eight miles. I was told in 1878 that the rifts seen on each side of the northern end of the lava extended its whole length, but this is not the case.
Leading our ponies, we followed the western rift southward between it and the lava. It is somewhat similar in appearance to the well-known Allmen's-rift and Eaven-rift, near ringvdlir, but narrower and far deeper; its depths not having been filled in with debris and soil, as is the case with the two mentioned. A quantity of snow, remaining from the preceding five winters, lay at the bottom of the rift, no ray of sunlight ever penetrating there. It cannot longer be doubted that the Allmen's-rift and Eaven-rift were formed by the breaking away and subsidence of the tract between them, in the same way as these two recently-formed rifts in the Orsefi.
The lava now forms a bed tending from S.S.W. to N.N.E., varying greatly in breadth, and said to be over twenty miles in length. To judge from its appearance, a vast quantity, most intensely heated, must first have welled forth from a rift running down the centre of the bed for nearly its whole length. This, owing to the comparatively level nature of the ground, spread freely westward and eastward wherever slight depressions existed in the j)lain of the (Jrseji. In places these arms cover tracts several square miles in extent The largest is on the western side, about the centre of the bed, and covers an area of some six or seven square miles. This first eruption of molten rock congealed into a rugged sheet, twelve to fifty feet in thickness, very clinker-like along its borders, which are fragments of the earlier cooled fringe of the molten flood that were borne forward by that behind. It is evident from the veritable chaos of huge masses of lava piled up in places down the centre of the bed, above the rift whence" the lava issued, that eruption after eruption took place, each lava-flow being congealed into a layer of rock, which in its turn was subsequently upheaved and shattered by a later outburst, in whose fiery embrace the huge jagged masses of the torn-up bed were borne along partly embedded, and from which, now that the later of those molten floods are also solid rock, they project at all angles. These masses are also built up into several groups of rude cone-shaped craters having an altitude of from one hundred to two hundred feet.
By the aid of my hands, protected by thick woollen mittens that they might not be cut by the lava, I made my way with some difficulty across to a group of craters thus built up, at a spot about a mile and a half from the northern end of the lava, to get an idea of the extent and general appearance of the bed, and ascertain whether the lava had issued, as I surmised, from a continuous rift, or from older volcanic cones, a number of which stud the Orsefi. I had surmised rightly, for nowhere could I see evidence of any molten matter having streamed down the outer walls of the craters, or that the craters themselves were older than the lava. They appeared to have been merely vents maintained through the lava-flows at intervals above the rift for the escape of heated vapours from below, in which, at times, molten matter had risen a few feet ere it found an outlet at a lower level; which the later lava-flows appear to have done by forcing their way under and through the congealed earlier ones in a most extraordinary manner, reducing the under surfaces of the older, wherever they had been in contact with the fiery floods, again to a molten state, and upheaving immense masses. Eadiating from the craters, I also noticed numerous deep fissures formed by the contraction of the lava while cooling, and deep abysses, plutonian in their gloom, where huge slabs had fallen in after the molten lava had flowed away from beneath. No heated air was ascending, so that in five years the whole mass appeared to have quite cooled Owing to the erratic manner in which the lava has spread around and been piled up down the centre of the bed, it is not an easy matter to compute the cubic contents of the enormous molten mass that here issued in 1875; but I believe I shall under rather than over estimate if I take the length of the bed at twenty miles, and give it a mean width of five miles- the vast sheets forming the arms being allowed for those parts of the main bed of far less width, and a thickness of one hundred feet-not too great a thickness when the vast quantity piled down the centre of the bed is taken into consideration, as well as the fact that all depressions in the area covered have been filled in; and thus computed it amounts in round numbers to over thirty-one thousands of millions of cubic feet. This lava-flood, therefore, is as nearly as possible twice as large as that from Eekla in 1845, which has been computed to contain 14,400 millions of cubic feet (Danish), and it dwarfs into- insignificance the lava-streams ejected during the eruptions of Vesuvius in 1794 and 1855, which have been computed at 730 and 570 millions of cubic feet respectively. I believe that only once since the settlement of Iceland has a larger one flowed forth, and that was in 1783, from rifts which opened in, and north of the Varmdrdalr (Warm-river-valley) at the foot of the SJcaptdr Jokull, on the west side of the Vatna. Burton and other writers state that this lava flowed from the SJcaptdr Jokull; but an Icelander, Herra Thoroddsen, who contributes a paper on the Icelandic Volcanoes to the October number of ' The Geological Magazine' says : ' These eruptions are erroneously stated to have taken place in Skaptdrjokull, where an eruption has never yet occurred.' But what is remarkable, Herra Thoroddsen, with strange inconsistency, in the same paper credits the Jokull, under its other name of Sidu Jokull, with an eruption in the year 1389, and one in its neighbourhood in 1753.