472 E. B. Bailey — Iceland, a Stepping -Stone. 



rather than of observation. This cannot be said, however, of the 

 strato-volcano Hekla(l,557 m. high), which extends 27km. parallel 

 to the local fissures, and only 2-5 km. at right angles to them. 

 Regarding the Kilauean domes, where the difficulty of interpretation 

 is admittedly great, Thoroddsen summarizes his position as follows : 

 " The geological evidence shows above all that the Icelandic domes 

 have been built on fissures where the eruption canal has been opened 

 so wide as to give unimpeded egress to the lavas." Askja, with its 

 caldera 55 sq. km. in extent, he points out, is situated at the 

 crossing of two major fissure-systems ; and he indicates on his map 

 that its caldera is margined to some extent by a crater- row 

 (Spethmann on his large-scale map shows additional rim-craters and 

 solfataras). He further mentions an explosion-crater as having 

 arisen on an obvious open fissure near the edge of a considerable 

 subsidence which in 1875 affected about a quarter of the bottom of 

 the Askja Caldera. 



The 1875 eruption at Askja, and another of earlier date, 1724, at 

 Viti, are the only two known to have given rise to explosion craters 

 during historic times. Both eruption centres produced nothing but 

 ash ; but the relief of pressure in each case seems to have upset the 

 equilibrium of neighbouring districts, leading to great subsidence and 

 Assuring, and to an immense outpouring of lava. 



Viti enjoys the rare distinction of being to all appearance 

 unconnected with any fissure that has reached the surface. It has 

 built but a small cone, so that the surrounding country is open to 

 observation. 



3. St/ess on Lunar Craters and Terrestrial Arcs. 



In the paper already referred to, read before the Geological 

 Society of London, Clough, Maufe, and the present writer suggested 

 an analogy "between the Glen Coe cauldron of Old Red Sandstone 

 times, with its girdle of fault-intrusion, and the Pacific Ocean of 

 to-day, with its fringe of marginal volcanoes". The source of 

 inspiration is not far to seek. While we were mapping in Glen Coe 

 we felt as though we were reading Suess again, and that our 

 discoveries had already been in large measure described in vol. i of 

 that author's masterpiece. 



Naturally, on the appearance of vol. iv of the Sollas edition of 

 The Face of the Earth, 1 we turned with keen anticipation to those 

 passages in which Suess develops his earlier conceptions of cauldron- 

 inbreaks and arcuate structures in general. We found there an 

 introduction to Thoroddsen's work on the ring-fractures of Iceland, 

 dealt with in Part 2 of this communication, and also much 

 additional matter which serves as the basis of the discussion that 

 follows. 



After recalling how, at the conclusion of vol. i, he had already 



1 The many page references that follow all refer to this volume. In 

 its footnotes the original sources may often be traced. For additional 

 information, including several very helpful illustrations, the reader may 

 advantageously consult the corresponding parts of tome iii of the de Margerie 

 edition,- La Face de la Terre. 



