on the principal Gey sirs of Iceland. 393 



the Strokkur, I shall describe in a summary manner the topo- 

 graphical and geological position of tl^ese two springs, for the 

 geological portion of the Voyage en Islande of the Scientific 

 Commission of the North contains extensive details upon 

 this subject. 



The two principal geysirs of Iceland are situated in the 

 south-east part of the island, at eighty-seven kilometres, in a 

 straight line, to the east-north-east of Reykjavik, and at forty- 

 six kilometres to the north-west of Hecla. These fountains, 

 accompanied by a gi'eat number o{ hvers and langs, occupy a 

 space of 500 metres from north to south, by 100 metres from 

 east to west, towards the northern part of a large plateau of 

 volcanic tufa, bordered on three sides by large glaciers or 

 JoJculls, the principal of which are, — on the north, the Lang- 

 JcikuU; on the north-east, the Hofs-Jokull and the Arnarfells- 

 JokuU; on the east, the Skaptar-Jokull ; on the south, Hecla 

 and the Torfa-Jokull. 



This vast plateau is traversed, from north-east to south- 

 west, by two of the largest rivers in Iceland, the Hvita and 

 the Thjorsa, and by their numerous tributaries: these two 

 principal rivers, which take their sources, the first in the 

 snows of the Lang-Jokull, and the second at the foot of the 

 Arnarfells-Jokull, flow into the sea at the south-west part of 

 the plateau, which is terminated on this side by low sandy 

 shores. At numerous points of the plateau containing the gey- 

 sirs, rise mountains which have in many instances the form of 

 isolated cones, composed of a conglomerate, the chief con- 

 stituent of which is basalt and a brown tufa, the principal 

 element of which appears to be the mineral which Sartorius 

 von Waltershausen found in the ancient tufas of Etna, and 

 which he has named palagonite. The height of these moun- 

 tains, several of which have preserved the appearance of 

 craters of eruption, and still present, on their slopes or at 

 their base, debris of streams of lava, does not exceed 500 to 

 600 metres. 



Beside these cones, we meet with some hillocks of a gray 

 phonolite, inclosing numerous crystals of felspar, and very 

 small crystals of pyroxene: some strips of ancient trap or 

 compact basalt, often divided into columns, appear also at the 

 surface of the plateau; and a sort of vitreous pitchstone, both 

 green and reddish, which appears to have resulted from the 

 fusion of the phonolite by the basalt, is ordinarily seen in 

 contact with these two rocks. 



The hill nearest to the geysirs, from which it is distant 

 only about 3(i0 metres, to tlic west, consists of upheaved 

 and almost vertical strata of a bluish gray phonolite, much 



