REPORT OF THE CHIEF ASTRONOMER 707 



SESSIONAL PAPER No. 25a 



it, so that henceforth Kilauea and Mokuaweoweo are independent. As yet 

 this suggestion cannot be proved, but it has weight enough to indicate the need 

 of caution in drawing conclusions regarding the non-existence of a fluid sub- 

 stratum beneath the island of Hawaii. Similar care should be taken in discuss- 

 ing the independent levels of lava in other pairs of volcanoes. 



The conditions of abyssal injection have been partially discussed by the 

 writer in a special paper.* Its aim was to state the consequences of the doctrine 

 of the ' level of no strain ' in a cooling earth, as these bear on magmatic intru- 

 sion. Abyssal injection was explained as due to the peculiar state of the ' shell 

 of tension,' which because of its very nature permits of ready splitting by 

 intruded magma. While many facts seem to agree with the hypothesis, several 

 of its premises are unproved; so that now, as at the time of publication, the 

 idea is in the writer's mind largely a matter of pure speculation. The advantage 

 of such speculation is that it sharpens the scrutiny of the hypothetical premises. 

 These are the subject of observation and experiment, each of which must show 

 improved results as all alternative explanations are kept in mind during an 

 investigation. Meanwhile, explanation is not proof, and geologists must be 

 content, for a time, to regard abyssal injection as a fact, mysterious as it is 

 true. (See pages 572 to 575.) 



Each abyssally injected body represents an ' intercrustal magma-basin.' In 

 general, the ' shell of compression ' will not readily allow of the passage of 

 magma to the earth's surface, so that the magma would often rise but little 

 above the ' level of zero strain.' The injected body retains physical connection 

 with the substratum and stands above it in vertical form, like a dike, though 

 the size may be batholithic in many cases. Such bodies are conceived to be the 

 proximate sources of the extrusive and intrusive rocks with which the field 

 geologist has to deal. 



Origin of Volcanic Action. 



Volcanic action may be defined as the working of the extrusive mechanism 

 which brings to the earth's surface rock-matter or free gas, initially at the 

 temperature of incandescence. The mechanism includes: the localization, open- 

 ing, and shaping of the vent; the persistence of a vent as an open channel 

 during seconds, days, years, or milleniums; the conditions for lava outflow, for 

 gas and vapour outflow, and for the separation of gas or vapour from lava; the 

 conditions leading to the periodicity of eruption at central vents; and those 

 leading to chemical variation in the erupted magma. This section is intended 

 to sketch in very brief form a theory of volcanic action, founded on the principle 

 of abyssal injection. It is planned to develop the theory at greater length in 

 a separate publication. f 



According to the views expressed in modern text-books of geology, the 

 emission of incandescent matter at the earth's surface takes place either in the 



* American Journal of Science, Vol. 22, 1906, p. 195. 



f Issued, since forwarding the manuscript of this report, in Proc. Amer. Acad. Arts 

 and Sciences, Vol. 47, 1911, pp. 47-122. 

 25a— vol. iii — 46 



