CHAPTER V. 



SPECULATIONS CONCEBNING THE CAUSES OP VOLCANIC ACTION. 



The cause of the succession of rocks apparently a single phase of the more general cause of volcanism. — 

 The probable subterranean Jocus of volcanic activity. — Notion of an all-liquid interior. — Not asso- 

 ciated with volcauicity, and gives no explanation. — Large vesicles not tenable. — Localization of 

 volcanic phenomena. — Independence of vents. — Growth and decay of action. — Lavas not primor- 

 dial liquids. — Comparison of lavas with metamorphic rocks: First, with reference to chemical 

 constitution ; second, mineral components ; third, texture. — Possibility that lavas are remelted 

 metamorphic rocks. — All lavas cannot so originate. — Average composition of eruptive and sedi- 

 mentary rocks compared. — Agreement in composition between basalts and sedimentary rocks. — 

 Mr. King's hypothesis of segregation of crystals. — Primitive magma. — Conjectured source of 

 lavas. — Dynamical cause of eruptions. — Cyclical character of volcanism. — Elastic energy of erup- 

 tions. — Real nature of the dynamical problem. — The origin of the energy. — Increase of local sub- 

 terranean temperatures — Relief of pressure. — Access of water. — Linear arrangement. — Mechanics 

 of eruptions. — Penetrating power of lavas. — Expelling power. — Not effervescence, but pressure of 

 denser rocks overlying their reservoirs. — A simple application of hydrostatic laws. — Explanation 

 of the sequence of eruptions. — A compound function of density and fusibilty. — Graphical repre- 

 sentation. — Discussion of the hypothesis and objections to it. — Exceptions and anomalies. 



I have doubted the propriety of embodying' in a work devoted to a 

 statement of observed facts any views of a speculative nature. But the 

 representations of my director and associates have encouraged me to do so, 

 inasmuch as the subject is quite germane to the observations, and the ob- 

 servations are such as have stimulated great curiosity as to their causes. I 

 shall, therefore, present a trial hypothesis, which seems to me to explain the 

 sequence in the eruptive rocks now testified to prevail generally through- 

 out the Rocky Mountain Region. 



It seems as if the explanation of such an order of facts could only be a 

 phase of the more general cause of volcanism itself. But the origin of vol- 

 canic energy is one of the blankest mysteries of science, and it is strange 

 indeed, that a class of phenomena so long familiar to the human race and 

 so zealously studied through all the ages should be so utterly without ex- 

 planation. Nothing could be further from my intention than propounding 



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