59 



237 



vww^:^:\\vi 



thai the pressure of the steam can withstand the pressure of the water column in 

 the channel. A sudden increase of the heat supply will then give rise to an eruption. 



The principal fault of Mackenzie's theory is that it does not explain the 

 intermittence and periodicity of 

 the geysers, whicli are the most 

 characteristic features of the geyser 

 phenomena. 



Amendments of this theory, 

 proposed by later scientists, have 

 not, as far as I know, overcome 

 these difficulties in a satisfactory 

 manner. 



In order to see in what way 

 the existing theories need to be 

 amended in order to agree with 

 the existing facts, it will be very 

 instructive to make a closer 

 examination of an eruption of 

 one of the small geysers in its 

 different phases. I select as an 

 example the southern geyser at 

 Grafarbakki. 



I-ig. 14. 



Immediately after an eruption the small shallow basin is quite empty. The 

 dampness of the stones at the bottom of the basin is the only visible sign that 

 boiling water has recently filled the basin. But the water soon begins to show 

 itself between the stones at the bottom of the basin, and it rises steadily and rapidly. 

 At last it fills the whole basin and begins to overflow the brim. Shortly after, the 

 spring enters upon its next phase, when air bubbles commence rising through the 

 water from the bottom of the basin. The water is not boiling, for real ebullition 

 very seldom takes place in the Icelandic springs. The water is agitated by bubbles 

 of spring gas, which, saturated with vapour, force their way up through the water. 

 At first these bubbles are neither large nor numerous, but thej' increase rapidly 

 until at last they rise with such violence that the water is thrown up a little into 

 the air. The eruption has commenced. 



At the height of the eruption the water column is fairly constant, but it some- 

 times happens towards the end, when the energy of the eruption is declining, that 

 two or three water jets suddenly rise far above the others. It looks as if the motive 

 energy pulls itself together for a last effort. At the close of the eruption the water 

 in the basin sinks down again into the earth, preparatory to the next eruption. 



The main points of the mechanism of the periodic eruptions seem to be in- 

 cluded in the foregoing. I assume the supply of hot water, steam and spring gas 

 from the interior of the eartii to the lowest part of the spring channel, to be con- 

 st* 



