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CHAPTEK XIX. 



CONCOMITANTS AND RESULTS OF VOLCANIC ENERGY. 



BESIDES the igneous phenomena already described, there are some 

 other subjects, more or less associated with the geological history of 

 the earth and the reaction of the interior on the surface, which may 

 be termed concomitants of volcanic action. Among such are the 

 upheaval and depression of land ; breaks in the succession of strata, 

 which result from changes in relative level of land and water ; the 

 undulatory movements of earthquakes ; faults ; and the intrusion of 

 igneous matter into rocks in the form of dykes. It may be that 

 such phenomena are not always directly connected with volcanic erup- 

 tions, but no broad distinction can be drawn between those internal 

 movements in the earth which crumple strata, upheave mountains, 

 raise and depress continents, on the one hand, and the formation of 

 fractures through the rocks, by which vent is given on the surface to 

 the heated materials beneath, since both are consequences of contrac- 

 tions of the earth's crust. Hence we supplement the geological part 

 of volcanic history with the following considerations. 



Earthquakes. 



The study of earthquakes is the science of Seismology, which en- 

 deavours to trace the history and explain the origin of vibrations of 

 the earth's crust. 



Dr. Daubeny's Views. Dr. Daubeny regarded the primary shock 

 of an earthquake as the result of local volcanic excitement, evidence 

 of the accumulation and elastic pressure of imprisoned gases ; and the 

 propagation of the motion was attributed to wave-like vibration in 

 the mass of the rocks. 



Fissures in the rocks are occasioned by, and are the evidence of, 

 earlier convulsive movements. If we seek the cause of them, we 

 certainly find the greater part of the necessary evidence in the in- 

 numerable fractures and flexures of the earth's crust, of every geolo- 

 gical date, by which extraordinary disturbances of the strata have 

 happened ; for these very frequently occur in large areas, where no 

 other evidence of contemporaneous volcanic excitement can be dis- 

 covered. It follows obviously that many movements of the earth's 

 crust have been excited without the immediately preceding or coin- 

 cident local agency of volcanoes, as though all the differential effects 

 of volcanoes could be integrated into one energetic reaction of the 

 interior against the cooled and consolidated exterior crust. 1 



1 Humboldt's definition of volcanic agency in " Kosraos " contains this view. 

 VOL. I. X 



