476 E. B. Bailey — Iceland, a Stepping Stone. 



given in this connexion are legion, among them chapters vi and ix 

 of Daly's recent book Igneous Rocks and their Origin (1914). It 

 is not intended here to make a general survey of the subject, but 

 rather to sketch certain conclusions which seem to follow naturally 

 from Suess's presentation of this phase of world geology. The main 

 suggestions offered are as follows : — 



1. The arcuate form characteristic of many of the great mountain 

 chains can scarcely have originated through forward movement of 

 these chains, since it characterizes the Aleutian Isles, and other 

 typical examples, where the occurrence of forward movement has- 

 been denied. 



2. Iceland, with its volcanic arcs, large and small, may well 

 supply a stepping-stone between arcuate fractures of the type so 

 familiar in Scotland, with their accompaniment of ring-dykes, and 

 the vastly greater Aleutian Arc, with its serial volcanoes. 



3. The development of volcanic arcs suggests tension rather than 

 compression. The arcuate fractures of an asphalt pavement, the 

 circular cracks of a painted canvas, the miniature landslip fissures 

 on the slopes of a shell crater, all support this contention. 



4. Arcuate fractures have often served as guides for the uprise of 

 igneous magma. Volcanic accumulation is the surface manifestation j. 

 batholithic intrusion the corresponding subterranean phenomenon. 



5. Where, as in Iceland, the magma has been predominantly 

 basaltic, its mobility has favoured volcanic expression. "Where, as- 

 in the American Cordilleras, a vast supply of granitic magma has- 

 been drawn upon, or developed, its viscosity has determined intrusion 

 on a magnificent scale. The date and manner of origin of the 

 granite is not involved. 



6. The association of a folded Cordillera with a volcanic are 

 suggests the collapse of an unconsolidated axial batholith — a closing 

 together of its two walls and a corresponding compression of its 

 roof. By analogy the same suggestion holds even where the 

 volcanic arc is absent. 



In polar regions the cooling of an ice-flow often leads to a 

 development of tension-cracks. Into these, sea-water rises to be 

 frozen over at the surface. A return of warm weather brings the 

 walls together, and the roof of the temporary fissure is driven up te 

 form a ridge. The analogy between such a ridge and the Alps, 

 though obviously very imperfect, seems to merit a passing notice. 



7. The cause of a batholithic collapse need not be strictly local. 

 At Kilauea the solid floor of the crater, after ascending gradually for 

 years, will suddenly subside a thousand feet or so, in the course of 

 a few days. On occasion a volcanic outburst at some point on the 

 island accompanies this subsidence, but at other times nothing of the 

 kind is seen, and a submarine eruption is postulated. Perhaps 

 the outpourings of lava in the Brito-Icelandic province had a share 

 in the upheaval of the Alps. 



8. Once folding and thrusting have been developed, the outline 

 and almost every other feature of the mountain arc must be 

 profoundly modified. There is not only the revolution in what may 

 be termed the home country of the chain, but also the concomitant 



