230 52 



place during an earthquake. One lias therefore good ground for supposing that the 

 changeability of the hot springs at Reykjafoss is to some extent responsible for the 

 insignificance of the silicious sinters around the existing springs, as the deposit of 

 these sediments is a very slow process. The spring water must work in the same 

 place for a long period before leaving any visible signs of its activity. 



In contrast to the spring water with absorbed sulphuretted hydrogen, which 

 in Iceland breaks down all the rocks with which it comes into contact and causes 

 them to crumble, the alkaline spring water with absorbed carbonic acid gas seems 

 to smooth out its course and enclose it with a crust of silicious sinter. This pro- 

 cess possibly begins a little below the surface of the earth. If that be so, we can 

 still better understand why the alkaline springs at Reykjafoss have such small 

 stratifications, for by reason of the frequent changes in the course of the water, 

 the silicious compounds dissolved in the spring water are used up in enclosing the 

 course of the water through the crust of the earth. On the other hand, there is 

 in the neighbourhood of Reykjafoss a large number of silicious sinters dating 

 farther back. 



A very common feature of the springs belonging to the last-mentioned type, 

 is that most of them are periodic springs, and many of them are geysers. The 

 same is the case with the alkaline hot springs at Reykjafoss and at Hveravellir, 

 and very probably the thermal springs at "Störi Geysir" are of the same kind. 



There are only scattered and rather imperfect observations in existence con- 

 cerning the periods of these springs, so that at present one cannot affirm more 

 than that it is possible for springs belonging to the same group to have different 

 periods. Many examples of this can be seen at Hveravellir. For instance J>orvaldur 

 Thoroddsen' states that Nos. 15 and 16 have longer periods than the surrounding 

 springs. According to our less studied observations. No. 12 has a period of many 

 hours, while most of the other springs have periods of not more than a few 

 minutes. I had no opportunity of making a closer investigation on this point. 

 Most probably, however, the case is the same here as at the two geysers at Graf- 

 arbakki. In mentioning these geysers, f>. Thoroddsen - remarks that they spout 

 alternately, and that each eruption lasts for a minute. I made observations of the 

 eruptions of these geysers on the 5th. August 1906. The barometer stood at 757.4 mm. 

 and the temperature of the air was 13° Centigrade. 



These observations (see Table II) show a very regular period on the part of 

 the southern geyser. The pauses between the eruptions are very nearly equal, 608 

 seconds on the average. The measurements of the eruptions, on the other hand, 

 seem to indicate that they have a double period lasting about 44 and 53 seconds 

 alternately. But the measurements are too few to permit of a decision as to 

 whether this is due to a fortuitous circumstance or not. 



The figures for the northern spring do not show the same regularity, but as 



' loc. cit. 



- Geogr. Tidsskr. 10, 18, 1889—90. 



