DALY. — THE NATURE OF VOLCANIC ACTION. 67 



thermal gradient. That fact is inconsi.stent with the a-ssumption of a 

 thick roof over the granite batholith, and it must be remembered that 

 the batholith has been cooling for a length of time sulficient to permit 

 of the excavation of Goo-meter canyons in massive rhyolite. 



A brief study of the geysers thus lends much probability to these * 

 conclusions . first, that over thousands of S(|uare kilometers, the batho- 

 lithic roof was e.xtremely thin when the rhyolite plateau was formed ; 

 and secondly, that this thin roof was, in places, swallowed up, Briefly 

 put, the rhyolite of the plateau is locally bottomless, in the sense that 

 it passes downward into a typical granite batholith. The formation 

 of the rhyolite plateau means a kind of vulcanism which evidently 

 needs to be distinguished from fissure eruption and from ordinary 

 crater eruption. 



Central Eruptions. 



The larger part of volcanic literature deals with the activities at cone 

 and crater. Extensive and intensive as these studies have been, the 

 number of memoirs treating of all the essential problems of central 

 eruptions is very small. Yet every general theory of volcanic action 

 must undergo the test of such a thorough questionnaire. This applies 

 to the hypothesis that all vulcanism is a result of the abyssal injection 

 of primary basalt. The following argument will be clearer if a prelim- 

 inary list of the specific problems relating to central eruptions be 

 reviewed. The list includes : — 



1. The localization and opening of the vent. 



2. The persistence of a principal vent for many thousands of years. 

 8. The intermittent character of the eruptivity, including (a) the 



alternation of active and dormant phases, and {!>) the pulsatory or 

 gey.ser-like ([uality of eruption during the active stage. 



4. The origin of the heat which, by radiation in active craters, is 

 lost in stupendous quantities. 



.0. The normal evolution of a vent as illustrated in (ii) e.xplosiveness, 

 and (A) the nature of the lava emitted. 



6. The mechanism of lava outflow at central vents. 



These tests of our hypothesis will be briefly considered, nearly in the 

 order given. 



Opening and Ijtx'alization of the Vent. 



Evhirged fi!fmre.<<. — The events of 1 7H8 at the famous Laki fissure of 

 Icelaiul illustrate the close relation between some central eruptions and 

 the ])ronounced fracturing of the surface rocks of the earth. Kor much 

 or all of its length the master cra<'k was doubtless connected with atyp- 

 ical, narrow, abyssal injection. Many hills of the cone-and-crater type 



