Volcanic history frequently repeats itself. There had been no great eruption of Hecla for a period of about twelve years, and the people living in the neighborhood were congratulating themselves on the belief that the mountain was becoming actually extinct, and that therefore they need not trouble themselves about eruptions. Others, however, more farseeing, pointed out the fact that the lakes and rivers in the vicinity did not freeze, and that the amount of water they contained was greatly decreased.
The following description of the great eruption of Hecla that was remarkable both for its violence, as well as for the time during which it continued, is taken from Symington's "Sketches of Faroe Islands and Iceland":
"On the 4th of April, 1766, there were some slight shocks of an earthquake, and early next morning a pillar of sand, mingled with fire and red hot stones, burst with a loud thundering noise from its summit. Masses of pumice, six feet in circumference, were thrown to the distance of ten or fifteen miles, together with heavy magnetic stones, one of which, eight pounds weight, fell fourteen miles off, and sank into ground still hardened by the frost. The sand was carried towards the northwest, covering the land, 150 miles round, four inches deep, impeding the fishing boats along the coast, and darkening the air, so that at Thingore, 140 miles distant, it was impossible to know whether a sheet of paper was white or black. At Holum, 155 miles to the north, some persons thought they saw the stars shining through the sand-cloud. About mid-day, the wind veering round to the southeast, conveyed the dust into the central desert, and prevented it from totally destroying the pastures. On the 9th of April, the lava first appeared, spreading about five miles towards the southwest, and on the 23d of May, a column of water was seen shooting up in the midst of the sand. The last violent eruption was on the 5th of July, the mountains, in the interval, often ceasing to eject any matter; and the large stones thrown into the air were compared to a swarm of bees clustering around the mountain-top; the noise was heard like loud thunder forty miles distant, and the accompanying earthquakes were more severe at Krisuvik, eighty miles westward, than at half the distance on the opposite side. The eruptions are said to be in general more violent during a north or west wind than when it blows from the south or east, and on this occasion more matter was thrown out in mild than in stormy weather. Where the ashes were not too thick, it was observed that they increased the fertility of the grass fields, and some of them were carried even to the Orkney Islands, the inhabitants of which were at first terrified by what they considered showers of black snow."
The largest volcano in Iceland is Dyngjufjoll. This has on its summit the gigantic crater of Askja, some twenty-five square miles in area. This crater is of the intermediate form; the most general form of volcanoes on the island consisting of a number of craters that closely follow fissures.
Professor Johnstrup, in a report to the Danish Government, on this volcano, states that the valley of Askja has been gradually filled by repeated flows of lava from enormous craters on the edge of the mountain. In many places the surface of the earth is covered with bright red pumice stone that was thrown out during an eruption March 29th, 1875. Some of these craters are filled with steam that escapes with an almost deafening roar. The surprising feature of this eruption was the immense quantity of pumice stone that escaped.
The volcanoes in the Nyvatus Oraefi are entirely different. This barren plain is thirty-five miles in length and thirteen miles in breadth. Suddenly on the 18th of February, 1875, a volcano appeared in the centre, and four other craters were formed at subsequent dates. The mass of lava that was thrown out of these openings has been estimated at 10,000,000,000,000 cubic feet, or eighteen times the estimated mass of lava that has been emitted from Vesuvius between 1794 and 1855. This lava is basalt.
[CHAPTER V]
VESUVIUS
The old Greeks and Romans had but little knowledge of volcanoes. They only knew the volcanic mountains in the Mediterranean Sea. Here there are three volcanic regions:—one in the neighborhood of Naples; one including Sicily and the neighboring islands, and the other that of the Grecian Archipelago.