69 247 



of the gases from the same spring. If the composition of the spring gases is 

 changed in the course of time, then we have reason to suppose that a corre- 

 sponding change has taken place in the depth of the earth, where the spring has 

 its origin. 



For the purpose of comparison, I quote here Christensen's analyses of samples 

 of spring gases collected by Johnstrup at Krafla in the summer of 1871. 



We see at once that the last of Johnstrup's gas samples, (III), has almost 

 exactly the same composition as one of mine. The other two have a somewhat 

 more divergent composition, which is chiefly due to a comparatively large increase 

 in the amount of nitrogen. This suggests that these samples originate from springs 

 where the soil is particularly porous, and that a little atmospheric air has therefore 

 mingled with the spring gas. 



Unfortunately, however, there is no detailed description of the springs from 

 which the gas samples originate, in Johnstrup's and Christensen's papers. Any 

 conclusions drawn from a comparison of my analyses with those of Christensen 

 may therefore be to some extent uncertain. My two samples of gas show a con- 

 siderable divergence in their composition, although they originate from two very 

 similar hot springs, the distance between them being only about 20 m. Nevertheless 

 it is confirmed with sufficient accuracy by the analyses quoted, that the spring 

 gases at Krafla have undergone very little change, if any, in the period 1871 — 1906. 



We might expect that the gases evolved by the hot springs at Namafjall, 

 situated about 10 km. south of Krafla, would very closely resemble the gases 

 evolved by the hot springs at Krafla. The outward appearance of the springs is 

 in many respects similar, and the geological conditions indicate that the springs 

 originate from the same source. 



It was during the long period of volcanic eruptions 1722 — 1729, when Helvi'ti 

 was a very active ash-crater, and when the great line of craters at Leirhnji'ikur, a 

 short distance west of Helviti, emerged and threw up an immense quantity of lava, 

 that the active craters in Bjarnarflag, west of NamaQall, also came into existence, 

 (see Fig. 2). 



All this seems to indicate that the hot springs at Krafla and at Namafjall 

 must be in connection with each other. Nevertheless the composition of the spring 

 gases at Namafjall differs essentially from that of the spring gases at Krafla. The 

 great amount of hj'drogen in the former is especially conspicuous. 



