1909-] OUTLOOK OF SEISMIC GEOLOGY. 283 



chains of volcanoes in Mexico as mapped by Sapper^'^ seem to be 

 similarly associated with the great rift valley lying on the western 

 border of the Mexican plateau. 



It is in Iceland,' however, that the most extended studies have 

 been made of the most interesting field, in which the relation has 

 been worked out with the greatest thoroughness.^'^ Says Thoroddsen : 



One gains the impression that the form of the surface has no significance 

 as regards the volcanic force, which breaks out above upon the ridges, as 

 well as below in the valley, yet the volcanoes are always found associated 

 with areas which are either sinking or have sunk. 



The lava stream Ogmundarhraun in Krisnoik, which dates from about 

 1340, was poured out from two parallel clefts. The southernmost portion 

 of this stretch of country between the clefts after the beginning of the 

 eruption sank about 66 meters, and one side of the western fissure rose like 

 a vertical wall with four half craters open at the brink, the other halves 

 having sunk. At the end of the cleft is a visible dike which leads up to 

 the row of craters. 



Where great fractures or faults are present in the crust, the volcanic 

 forces have not always made a single passageway through them, but in the 

 vicinity on parallel clefts, often upon the high fracture margin; thus one 

 fracture line 50 km. long extends without volcanoes from Krisnoik to 

 Hengill, at which place the north side is sunk 200 to 300 meters ; parallel 

 with this is here found above at the margin of the cliff an almost uninter- 

 rupted series of craters which have formed not alone upon a single fissure 

 but over several slices and small fissures running parallel to one another. 

 A similar phenomenon is to be observed on the southern fracture margin 

 of the peninsula of Snaefellsnes where the craters are mainly found above 

 upon the edge of the bluff. Often, also, the reverse is the case, as for 

 example, in the Odadahraun, where the rows of craters for the most part 

 extend along the bases of the mountain chains, which rise as horsts from 

 the sunken ground on either side ; a like example occurs at Myvat'u, although 

 here the rows of craters occur at times above upon the ridge. 



In none of these cases have we evidence that the eruptions coin- 

 cided closely in time with the earthquakes which must have accom- 

 panied the movements of the earth strips between their bounding 

 faults, but the relationship of the one phenomenon to the other could 

 hardly be more clearly proven. Summing up the discussion, we 

 note that volcanoes, no less than earthquakes, help us to find the 

 positions of those fissures within the crust by which it is separated 



^' " Ueber die raiimliche Anordnung der Mexikanischen Vulkane," 

 Zeitsch. d. Deutsch. Geol. Gescll, 1893, pp. 574-577- 

 ''L. c, p. 3. 



