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SCIENCE 



[N. S. Vol. XXIX. No. 736 



sequent upon the adjustment of accumu- 

 lated stresses within the overlying rocks is 

 indicated by the sequence of fractures and 

 lava flows in the uppermost parts of the 

 earth, and the opening of eras of great vol- 

 canic activity after profound erogenic 

 movements have disturbed the compara- 

 tively quiet action of forces that have been 

 gradually shifting the stresses within the 

 outer portion of the earth. The magnitude 

 of the adjusting action is evinced by the 

 extent of territory simultaneously affected. 

 As, for example, the initiation of volcanic 

 action on a gigantic scale throughout west- 

 ern America at the end of Cretaceous time, 

 after an enormous period of nearly uniform 

 conditions of comparative quiet. 



The eruptive impulse, or energy, causing 

 the upward flow of magma, must originate 

 in the expansion of the magma upon relief 

 of pressure consequent upon the adjust- 

 ment of stresses in the overlying mass, and 

 from expansive energy of dissolved gases. 

 That the eruptive force is of nearly the 

 same order of magnitude as the stresses 

 within the earth's crust is shown by the 

 relatively small amount of material erupted 

 upon the surface of the earth compared 

 with the bulk of the whole; by the com- 

 mon intrusion of magma along fracture 

 planes and along those of structural weak- 

 ness, rather than at random through rock 

 masses; and most conspicuously, by the 

 evidence of equilibrium with the atmos- 

 phere maintained by lava in volcanic 

 craters. Open vents are known to exist 

 for centuries without great extrusion of 

 rock magma, as at Stromboli. The stresses 

 which produce condensation of volume in 

 proportion to depth and the results of ex- 

 pansion of volume are, therefore, some- 

 what evenly balanced. 



The efi'ect of expanding gases is shown 

 in the explosive character of many erup- 

 tions, and the periodic character of all 

 eruptions from open vents (volcanoes). 

 It must increase the volume of all magmas 



as pressure is relieved. Its effectiveness 

 must increase in proportion to the amount 

 of gas in the magma, which may result 

 from diffusion of gas from greater depths 

 of magma, and also from accession from 

 adjacent rocks under favorable condi- 

 tions. 



Spasmodic eruption may follow sudden 

 yielding of overlying rocks to long con- 

 tinued stresses, as in the case of massive, 

 or fissure, eruptions when there may have 

 been no considerable explosive action of 

 gas; or it may result from an accumulation 

 of gas pressure sufficient to rupture over- 

 lying rock masses. Eruption is then ac- 

 companied by abundant evidence of ex- 

 plosion. Both causes undoubtedly operate 

 together in most cases. 



In so far as magmatic eruption is a re- 

 sult of volumetric expansion of the magma, 

 due to relief of pressure, the shrinkage of 

 volume due to cooling will retard eruption, 

 or eventually stop it. Crystallization will 

 operate in the same direction. In pro- 

 portion as eruption is due to expansion of 

 dissolved gas, the escape of gas from 

 magma, or the reduction of supply, will 

 lessen the force of eruption, or eventually 

 put an end to it. The supply of gas from 

 great depths may be reduced by the grad- 

 ual diffusion of whatever is in a position 

 to be appreciably diffused; or the supply 

 from rocks adjacent to intruded magma 

 may be cut off by the closing of pores in 

 these rocks through metamorphism ; porous 

 rocks becoming dense and almost im- 

 pervious to gases. In these ways eruptive 

 action initiated by crustal readjustment 

 after continuing for variable periods may 

 come to an end. Readjustment of stresses 

 may recur from time to time in any region, 

 either at siich widely remote periods that 

 the volcanic activities associated with each 

 readjustment constitute distinct and sepa- 

 rate periods of action; or at such frequent 

 intervals that the results of several pro- 

 found movements are combined to form a 



