4. The Hofsjökull, including the Arnarfells branch to the east, and the Blágnýpujökull to the south-west. Occupying the centre of the island, it approaches the Túngnafellsjökull, an outlier of the Vatnajökull system to the south-east; and westward, it almost touches the north-eastern extremity of the long Reykjanes line.
5. The Hekla system, which the old theory of fissures connected with Etna. It lies on a parallel, a Palagonite ridge about 2000 feet high, extending from west to east through the Torfajökull, to the banks of the Skaptá.
6. The Vatnajökull, whose apex is Öræfa, the whole measuring some 330 miles in circumference, and occupying an area of 3000 to 4000 square geographical miles: stretches upon a parallel, and is connected by a meridian of lava-run with No. 2.
7. The Katla, or Kötlu-gjá system, again, is not linear, but disposed in a group at the southern extremity of Iceland. The principal items are the Mýrdals, Eyjafjalla, Merkr, Goðalands, and Tindfjalla Jökulls. This great mass is generally known as the Eastern Jökull, opposed to the Western or Snæfells.
8. The Reykjanes system is apparently the only diagonal which extends from the Fire Islands north-eastwards to Skjaldbreið, and to the snow mountains, whose northernmost point is Eyriksjökull. Its items are the Láng, the Ball, the Bláfells, the Geitlands, and the Ok.
Mr Keith Johnston, sen., and other authorities, give the following list of volcanic eruptions which have occurred during the present century.[69]
1. Aust-Jökull (an indefinite term for the great Eyjafjalla system), in December 1820 to June 1822, and January to June 1823.
2. Mýrdals Jökull (or rather Kötlu-gjá) in 1823, from 26th June, covered about a hundred square miles with sand and ashes.
3. Skeiðar Jökull began to erupt February 13, 1827, and did considerable damage. No record of this outbreak is to be found.
4. The submarine eruption off Cape Reykjanes took place in 1831.[70]