DALY. — THE NATURE OF VOLCANTC ACTION. 107 



"Without entering further into this subject, it will here suffice to 

 mention the principal causes for lava outtlow as deduced from the 

 abyssal-injection premise. These are: 



1. Very minute deformation of the feeding magma chamber. 



2. The effervescence of lava, due to the periodic accumulation 

 of magmatic gases in the vent. These gases may be juvenile or 

 resurgent. 



8. The assimilation of country-rock in depth, leading, probably, to 

 increase of volume. 



4. The increase of volume through heating in the conduit " furnace " 

 — a process specially like to occur during the dormant period when 

 the vent is temporarily plugged. 



These causes may co-operate, but at basaltic volcanoes the third is 

 clearly subordinate. 



The Two Types of Lava Flows. 



A preliminary study of the Hawaiian lavas, wilh respect to their 

 field habit, has led the writer to suspect significant gas-control even in 

 this detail of vulcanism. On the average the vesiculation of pahoehoe 

 or ropy lava was found to be more evenly developed than in the aa or 

 block lava. The contrast is illustrated in Plate V. The rather uni- 

 formly disseminated vesicles of pahoehoe are of relatively small and 

 relatively uniform size, and tend to have spherical form. The irregu- 

 larly distributed vesicles of aa lava are generally larger, though more 

 variable in size; much fewer in number, and of less total volume per 

 unit volume of rock ; and more irregular in shape. These facts indi- 

 cate a more uniform distribution of gas in the pahoehoe than in the 

 aa t}'pe. The aa vesicle, which is often thousands of times bigger than 

 the average pahoehoe vesicle, has undoubtedly grown through the 

 coalescence of many bubbles of gas. Such growth must in very high 

 degree (see page 78) favor the escape of the gas into the air, and we 

 may regard these large vesicles as representing so much gas trapped in 

 the freezing lava. Before solidification had set in, gas must have es- 

 caped from every aa flow in large volume. In fact, observers of the 

 two types in actual movement agree that the gas emanation from flow- 

 ing aa lava is much more abundant than that from flowing pahoehoe.*^ 



The difference of field habit in fluent lava and block lava is thus 

 explained, with some show of probability, by the relative abundance of 



*» Cf. J. D. Dana, Characteristics of Volcanoes, New Yorlc, 1891, p. 242, 



JudKf Hitchcock describes a typical Hawaiian aa flow as advaricititi "witli no 

 explosions, but a tremendous roaring, like ten thousand bhidt-furnaccs all at 

 work at once." 



