REPORT OF THE CHIEF ASTRONOMER 713 



SESSIONAL PAPER No. 25a 



gas-fluxed hole in the roof of a still-fluid laccolith, while its neighbour, Mokua- 

 weoweo (the main vent of Mauna Loa) is a ' principal ' volcano. Branco has 

 concluded that the 127 volcanic ' embryos ' of Suabia were vents from a large 

 laccolithic mass of late Tertiary age. Similarly, many of the Scottish necks, 

 made famous by Geikie's monographs, seem to represent late-Paleozoic out- 

 bursts of gas from thick sheets (sills and flat laccoliths). 



Some of the difficulties of volcanic theory fall away as soon as the distinc- 

 tion between the two kinds of central vents is clearly made. It helps us to 

 understand: the short lives of many volcanoes; the lack of lava flows at many 

 of them; the independent activity of neighbouring vents; the chemical dissimi- 

 larity of the lavas from neighbouring vents (each satellitic chamber pursuing 

 its independent chemical evolution along the lines of assimilation and differ- 

 entiation) ; the quite common clustering of many small vents in a region which 

 shows no trace or but few traces of alignment among its volcanoes; and the 

 frequent evidence of surface deformation in such regions. The evidences from 

 the existence of ' subordinate ' volcanoes are largely indirect but they are 

 numerous, and, taken together, they form a combination of no mean strength. 



Gathering all the threads of the argument just presented in skeleton outline, 

 we find them converging to one leading conclusion. The principle of abyssal 

 injection — intrusion of the substratum basalt along mechanically opened fissures 

 in the Earth's acid shell — seems to explain the essential facts of vulcanism. 

 The writer believes, in fact, that this fundamental postulate is as necessary to 

 sound theory in vulcanology as it is in purely plutonic geology. 



