282 HOBBS— THE EVOLUTION AND THE [April 24, 



directions are also the dominant ones in the fracture system of 

 North America.^^ 



A number of papers of a controversial nature have appeared 

 notably by Branca^* in opposition to the view that volcanoes are 

 aligned upon fissures, but inasmuch as they deal with districts in 

 which the evidence is more or less equivocal they need not be con- 

 sidered here. The problem of arrangement of volcanoes must be 

 solved not in southern Europe nor on the Mexican plateau border, 

 but in the volcano gardens of the world, such as Iceland or Java. 



Volcanic Extrusions in Relation to Block Adjustments. — Clar- 

 ence King in his description of the area included in the Fortieth 

 Parallel Survey,^^ an area divided by vertical faults into great blocks 

 which underwent adjustments at the close of the Miocene, has 

 furnished a classical instance of the relation of volcanic outflow of 

 lava to block movement. He says : 



Single ranges were divided into three or four blocks, of which some 

 sank thousands of feet below the level of others. The greatest rhyolite 

 eruptions accompanied these loci of subsidence. Where a great mountain 

 block has been detached from its direct connections and dropped below the 

 surrounding levels, there the rhyolites have overflowed it and built up great 

 accumulations of ejecta. Whenever the rhyolites, on the other hand, accom- 

 pany the relatively elevated mountain-blocks, they are present merely as 

 bordering bands skirting the foothills of the mountain mass. There are a 

 few instances in which hill masses were riven by dykes from which there 

 was a limited outflow over the high summits — but the general law was, 

 that the great ejections took place in subsided regions. 



The study of the great rifts of eastern Africa seems to have 

 shown that the volcanoes which have there been built up, are simi- 

 larly related to the sinking of the great strips of country which have 

 caused the chief inequalities of the general surface.^® The two 



^' Hobbs, " The Correlation of Fracture Systems and the Evidences of 

 Planetary Dislocations within the Earth's Crust," Trans. Wis. Acad. Sci., Vol. 



15, 190S, pp. 15-29. 



^^W. Branca, " Zur Spaltenfrage der Vulkane," Sitsungshcr. Ak. Wiss., 

 Berlin, 1903, pp. 748-7S6. 



*°" United States Exploration of the Fortieth Parallel," Vol. i, Sys- 

 tematic Geology, 1878, p. 694. 



^'Ed. Suess, "Die Briiche des ostlichen Afrika," Denksch. Weincr Akad., 

 Math. Naturzv. KL, Vol. 58, 1891, pp. 555-584- 



