108 PROCEEDINGS OF THE AMERICAN ACADEMY. 



volatile matter and, still more, by the evenness of its distribution. 

 For both reasons pahoehoe lava is certain to be less viscous than is aa 

 lava, other conditions being the same ; the pahoehoe moves, as it were, 

 on molecular and vesicular "ball-bearings." 



The fact that many flows, fpom the very points of emission, are al- 

 together of the one type, while others are throughout of the other t}^e, 

 shows that the differences of gas-distribution are developed in the vent. 

 The problem as to exactly what circumstances there control the gas- 

 distribution has not yet been solved. Slight differences in temperature, 

 or differences in the advance toward solidification (with gas expulsion) 

 may be the effective cause. The writer has observed a tendency for 

 the phenocrysts of aa lava to be of larger average size than those 

 in pahoehoe lava which gives practically the same oxide proportions 

 in ordinary chemical analysis (volatile matter other than water neg- 

 lected) ; but he is as yet not prepared to regard this as an established 

 rule. 



Vulcanism Originating in Satellitic Injections. 



We have so far considered central vents as, in general, direct off- 

 shoots of main abyssal injections. The latter have been described: 

 as dike-like, though often of great widths ; as extending upwards from 

 the primary substratum, nearly or quite to the earth's surface for some 

 such vertical distance as forty kilometers. Batholiths have been in- 

 terpreted as chemically modified abyssal injections of the primary basalt. 

 Plutonic stocks and bosses represent cupolas in batholithic roofs. Stocks, 

 bosses, and batholiths compose the group of "subjacent" intrusive 

 bodies. ^'^ Laccoliths, sheets, and ordinary dikes are individualized 

 bodies, satellitic with respect to their feeding abyssal injections, and 

 like the latter, owe their intrusion to a simple parting of the invaded 

 rock-formations. Irregular bodies intruded in the same fashion have 

 been called " chonoliths " : they form a fourth class of "satellitic 

 injections." ^^ 



All satellitic injections soon lose thermal and hydrostatic connection 

 with their respective abyssal injections. All laccoliths and chonoliths, 

 like most sheets and some dikes, have solid floors during most of their 

 magmatic activity. If a satellitic injection is of large size, its content of 

 heat energy and of gas may suffice to open one or more vents to the 

 earth's surface, according to the methods already described. Volcanic 

 action is thus initiated which differs in some respects from that due to 

 direct emanations from a main abyssal injection. 



" R. A. Daly, Journal of Geology, 13, 508 (1905). 

 »i Cf. Journal of Geology, 13, 498 (1905). 



