480 HARRY 0. WOOD 



hypothesized. The lips of the rift were not outforced or uplifted, 

 nor were radial cracks produced. Such action of lava and gas out- 

 rush as was observed would be expectable if eruption were permitted 

 through tectonic rending of the mountain shell in a manifestly 

 weak zone, thus unsettling the physical-chemical equilibrium of the 

 gas-charged magma afforded exit to the surface and the atmosphere 

 in this way. Many of the eruptions of Mauna Loa have exhibited 

 similar peculiarities, perhaps most of those observed. This whole 

 aspect of eruption here deserves a much more thorough discussion 

 than can be given it in this paper. However, to the mind of the 

 writer, the phenomena of the 1916 eruption appear to illustrate 

 admirably the conception so clearly stated by Geikie, 1 if only the 

 expression "tectonic strain" be substituted for his phrase "terres- 

 trial contraction," with the emphasis in this instance on tectonic 

 strain as a cause. 



The fact that some of the circumstances of this eruption might 

 be interpreted adversely to this view may be discussed more 

 advantageously in a systematic study of the seismic accompaniment. 

 Altogether this conception deserves careful attention. Of course, it 

 must not be considered to invalidate or displace views developed 

 from, and applied to, other modes of eruption in other lands, but it 

 must not be rejected simply because it is different from them. 

 Moreover, there is no disposition to overlook the very real applica- 

 tion of more commonly recognized modes of eruption in this recent 

 outbreak, or in other eruptions in Hawaii. It is intended merely 

 to give this tectonic mechanism emphasis and to point out its 

 possible, or probable, local predominance. 



Lava did not well out of the new rift along its whole length. 

 Hence we may subdivide it, recognizing two segments: a longer 

 flow-source segment, and a shorter solfatara-spatter-cone segment 

 stretching on up the mountain beyond the head of flow. 



THE FLOW-SOURCE SEGMENT 



For about 2\ miles from Puu o Keokeo in a direction about 

 N. 3 W. mag. the new rift is indicated by a system of open fissures 

 straddled by seven double semicones, varying from 20 to 100 feet 



1 Ancient Volcanoes oj Great Britain, I, 10-13. 



