706 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



enough to flow at low pressure, is everywhere present at moderate depths below 

 the earth's surface, we are in accord with the nearly unanimous opinion of 

 geologists. If the observed temperature gradient is specially steep because of 

 the concentration of radioactive matter in a thin surface shell, the thickness 

 of the crust may be as much as 100 miles or more. In any case, the depth of 

 the bottom of the crust is probably much greater than the depth of any 

 exposed intrusive contact at the time of the intrusion of that body. The magma 

 must penetrate at least 15 or 20 miles of crust before it reaches such levels 

 as those registered in the known, actually seen, igneous bodies. 



How this penetration of the lower and thicker part of the crust takes place 

 has always been a difficult problem. The idea that the primary rock-magma 

 melts its way to the surface, or even to the levels of now visible intrusive con- 

 tacts, may be dismissed. The very great superheat demanded can scarcely be 

 admitted for an earth-shell so close to the surface. The only alternative seems 

 to be the usual conception that the magma always traverses the lower and thicker 

 part of the earth's crust along mechanically opened fissures. To this process 

 the name ' abyssal injection ' may be given.* It is to be regarded as the prelude 

 to vulcanism, or to intrusion, whether of laccoliths, dikes, or batholiths. 



For the shell of eruptible rock-matter we have the old, appropriate name 

 * substratum,' as employed by Fisher, Lowthian Green, and others. In most prob- 

 lems of igneous geology it is not necessary to decide on the question as to the 

 rigidity of the substratum with respect to such cosmic forces as the earth tides. 

 Since, however, the latent heat of crystalline silicate rocks is about one-fifth 

 of their total melting heat when just molten, the simplest supposition is that 

 the substratum is not crystallized. The transformation of a crystalline sub- 

 stratum into fluid magma at the lower openings of abyssal fissures is evidently 

 much more difficult than the change of an isotropic, highly rigid liquid into a 

 readily eruptible, distinctly fluid magma. The many attacks on the hypothesis 

 of a liquid substratum have failed to disprove it, because there has been general 

 neglect of the view that, under great pressures, liquid rock, though very hot, 

 may rival crystalline rock in rigidity. 



The idea of a fluid substratum has often been dismissed by authors because 

 of the observed independence of the vents during the simultaneous activity of 

 Kilauea and Mokuaweoweo (Mauna Loa), in Hawaii. It is held that, if the two 

 lava columns rise from a common liquid substratum, the level of the higher 

 column must, by simple hydrostatic action, be kept at the general level of the 

 lower. The actual equilibrium is kept with one column some 9,000 feet taller 

 than the other. Attempts have been made to explain this contrast in levels by 

 a difference in density of the two columns; but there is nothing in the known 

 facts to uphold the suggestion. On the other hand, the field evidence in Hawaii 

 favours the belief in the present independence of the two vents. There is 

 something to be said for the hypothesis that the lava pit at Kilauea is the open- 

 ing in the roof of a large laccolith, which has been injected into the old lavas 

 of Mauna Loa. The freezing of the dike feeders of the laccolith would isolate 



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



