650 J REPORT— 1894. 



7. On Certain Volcanic Subsidences in the Worth of Iceland. 

 By Tempest Anderson, M.D., B.Sc, F.G.S. 



Perliaps the most striking features in Icelandic scenery are the gids (pronounced 

 *geow'),or fissures and chasms which are so frequently met with in all the districts 

 in which recent volcanic activity manifests itself. They are usually, and in most 

 cases rightly, ascribed to the lower stratum of a molten lava stream having obtained 

 an outlet after the surface has consolidated into a crust of greater or less thickness. 



Gias of this class are, so far as the author has been able to observe, confined 

 within the limits of a single lava stream, and do not aifect previously formed rocks. 

 Usually there is a large gia roughly parallel with each side of the original lava 

 stream, and the space between these has subsided considerably. Any gias in this 

 subsided portion are much smaller, and obviously of secondary importance. 

 E.tamples of this are to be found in the well-known Almanagia, at Thingvalla, 

 which has a throw of about 100 feet, while the sides of the smaller gias which 

 enclose the Logberg in the subsided portion are practically ^on the same level. 



There are also several such subsidences near Lon and Asbergi, in the North of 

 Iceland. The main subsidence at Asbergi is a little more compUcated, though 

 evidently due to the same causes. Here a large roughly triangular area has sub- 

 sided, the throw at the apex being probably nearly 300 feet, but a space in the 

 middle has remained at its original height, so that a depression has been produced 

 like a great V> the portions both between and outside the legs having remained 

 standing. In the case of Thingvalla it appears not unlikely that the lava which 

 flowed down into the lake solidified on coming in contact with the water and 

 formed a wall sufficiently strong to hold up the lava plain till it formed a firm 

 crust, and that the giving way of this and the escape of the molten lower layers 

 into the deeper parts of the lake caused the^subsidence. 



Similarly the lava which escaped from Asbergi may have been that which now 

 occupies the low ground near the estuary of the Jokulsa, in the direction of Lon. 



On the east and south-east of Lake Myvatn a very extensive eruption, or series of 

 eruptions, has taken place from a chain of craters locally called Gardr Borgir (' the 

 castles of Gardr,' which is the name of a farm). The lava How has occupied nearly 

 all the bed of Lake Myvatn, and flowed down the valley of the Laxa to its mouth 

 at Laxamyri. All this stream of lava is very remarkable for the number and size 

 of the spiracles with which it is studded, and a regular gradation of sizes exists, 

 between spiracles the size of a haycock and cones some of which cannot be less 

 than 200 feet high. These cones and craters, which constitute such a striking 

 feature of Lake Myvatn, may probably be nothing more than spiracles formed by 

 the escape of steam generated by the contiict of the hot lava witli the water of the 

 lake. The barrier which holds up the water of the present lake consists of this 

 lava, and caves exist in it which are obviously channels by which molten lava has 

 escaped. These and deeper-seated ones would be those by which the lava escaped 

 and left the depression occupied by the present lake. Between the craters of 

 eruption and the lake no spiracles were noticed, but there is a very remarkable 

 series of rocks— the Dimmuborgir — masses of lava of fantastic shape, 30 or 40 feet 

 high, which have remained standing while the intervening portions have subsided. 

 They present slickenside marks where the subsiding portions have scratched the 

 masses that have remained standing, and tide-marks where the crust has halted in 

 its descent ; also in many places bulgings, where the lava has been scarcely stiff 

 enough to stand, and others where it has actually formed stalactitic masses. 



So far for actual lava subsidences. 



The special object of this paper is to draw attention to a subsidence on the 

 slopes of Leirnukr, a volcano several miles north of Myvatn, where a large strip 

 of land, perhaps 200 yards wide and one mile or more long, has been let down to a 

 varying depth, averaging perhaps 60 to 80 feet. 



The faults bounding it, like nearly all the fissures in this district, run north 

 and south ; and the east face, which is most perfect, cuts right through a thick 

 stream of old columnar lava and through a large boss of tuft', round and over which 

 the lava has bedded itself, and also through the tuff' rocks at each side of the lav* 



