668 



MESSES. CLOTTGH, MA.TJFE, AND BAILEY ON [Nov. I909,. 



great eruption took place within the Askja cauldron. The Rudloff 

 Crater was established by explosion, and a great mass of pumice 

 and obsidian hurled forth over the surrounding country. Later, 

 between the visits of "W. L. Watts in the summer of 1875 and of 

 Jon Thorkelsson in January, 1876 (Spethmann, op. tit. p. 423) r 



Eig. 13. — Sketch-map of the Askja Caldera in Central Iceland, 

 after Spethmann, on the scale of about 1 : 90,000. 



lit. 



'////A 



the Knebel Lake depression, measuring a mile and a half across,. 

 was formed by subsidence in the floor of the Askja cauldron. 

 The depth of the hollow is not known with certainty ; but, when 

 Johnstrup visited the place in the first year of its existence, he 

 found that the surface of the water in the lake was still 740 feet 

 below the floor of the surrounding Askja caldera. The water was 

 at this time quite hot (40° C), having been largely supplied by 

 solfataras. 



Spethmann points out the obvious connexion between the 

 eruption of the Eudloff Crater and the formation of the Knebel 

 caldera. At the same time he rather lays stress upon the eruption 

 as the cause of the subsidence, since it preceded the latter by an 

 interval of some few months. It is possible, however, to regard 

 the lack of strict synchronism, in this case, as a minor phenomenon, 



