230 THORODDSEN 



spouting, (2) intermittently spouting springs, (3) alternately spouting 

 springs, (4) constantly boiling springs which do not spout, (5) springs 

 with a high temperature and a quiet surface or which boil quietly 

 in the middle. All the boiling springs deposit siliceous sinter. The 

 most famous hot spring is Geysir, in the vicinity of Haukadalur in 

 South Iceland, in the centre of a group of other boiling springs. 

 This group of springs was mentioned for the first time in 1294 and 

 has often undergone alteration by earthquakes, especially in 1630 

 and 1789. Geysir's eruptions now take place very irregularly and 

 many days may intervene between them. At the end of the 18th 

 and the beginning of the 19th century, Geysir's activity was at its 

 maximum, and it could throw up fountains of water to a height of 

 50 60 metres, w r hereas now the water rarely rises above 30 metres. 

 The neighbouring hot spring Strokkur, began its activity in 1789 

 and ceased during the earthquakes of 1896. At first Strokkur threw 

 up higher fountains of water than did Geysir, not only boiling water 

 and steam, but also cold water. Other large groups of hot springs 

 are found near Revkir in Olfus, in Revkholtsdalur, on Hveravellir 



, , 



on the plateau NE. of Langjokull, and in many other places. 



Sulphur springs occur in abundance in the volcanic districts, 

 but not outside the palagonite-formation; also alkaline springs are 

 common in the basalt districts. The alkaline springs are found upon 

 the level land, in valleys and upon mountain-sides where there is 

 much underground w r ater, but the sulphur springs commonly occur 

 upon mountain ridges and other dry localities where the water has 

 an outlet through the underlying lava, etc. Several of the solfataras 

 deposit a considerable amount of sulphur in small heaps w r here 

 the sulphurous vapours rise from the soil. Sulphur from Iceland 

 had a commercial importance even in the 13th century, and the 

 trade in sulphur w r as especially lucrative in the 16th century. Since 

 that time the export of it has gradually decreased and now has 

 entirely ceased. The sulphurous vapours which rise through the 

 clefts and cracks in the earth have a great effect upon the neigh- 

 bouring rocks, which are transformed and decomposed in various 

 ways coloured clays, gypsum, iron-alum (Halotrichit), etc. being 

 formed. The mountains which have been penetrated by the hot 

 sulphurous vapours are easily recognizable at a long distance, owing 

 to their naked and discoloured appearance; they arc always light- 

 red, yellow and white in colour and are entirely destitute of plant- 



