DALY. — THE NATURE OF VOLCANIC ACTION. 119 



The chief object of the foregoing discussion has been, however, not 

 to propose a division directly useful in field work, so much as to erect 

 a fence over which speculation about the earth's interior cannot pass. 

 The best way to check mischievous speculation is to advance bene- 

 ficent speculation, founded on all the known facts. For example, the 

 bold statement that there can be no magmatic substratum beneath 

 a district bearing two simultaneously active lava columns of ditfering 

 heights can no longer be made without an investigation of their nature 

 as " principal " vents (abyssal injections) or " subordinate " vents 

 (satellitic injections). The formal classification is of positive use in 

 recognizing a mechanism by which the petrographic contrast of the 

 lavas from neighboring vents may have originated. If two of these are 

 opened above different satellitic injections, the chances are good that 

 the magmatic histories of the injections will be different ; their emanat- 

 ing lavas would diverge in chemical type according to the progress 

 respectively made in the formation of syntectic and differentiated 

 magmas. 



Some " subordinate " vents are monogenetic in Stiibel's sense, and 

 there are a few analogies between the system of vulcanism here sug- 

 gested and that elaborated by the illustrious German. But the writer 

 cannot agree with Stiibel's principal conclusion as to the motor power 

 in vulcanism, and entirely fails to find geologic or petrologic evidence 

 for the existence of his " Panzerdecke." 



General Summ.\ry. 



The general h}T)othesis briefly outlined in the present paper assumes, 

 by sanction of the facts of general geology, that the earth is exteriorly 

 composed of successive shells of density increasing with depth. Be- 

 neath the interrupted sedimentary shell is a continuous solid "gran- 

 itic " shell, and still deeper, an eruptible basaltic shell or substratum. 

 All igneous action, since an early pre-Cambrian period, is the result of 

 the mechanical intrusion of the substratum basalt into the overlying 

 shell. This fundamental process is specifically called " abyssal injic- 

 tion." It is not a hypothetical process, but one which is clearly ap- 

 parent in the chemistry and field relations of igneous rocks. The 

 conditions leading to abyssal injection form a subject of great theoret- 

 ical difficulty, but the discovery of the e.xact medianism is not essen- 

 tial to the jjresented explanation of volcanoes. Nor is it necessary to 

 decide on the degree of viscosity characterizing the basaltic substratum, 

 although it is pointed out, once again, that the observed small amount 

 of deformation of this planet under cosmical stresses does not prove 



