R. A. Daly — Abyssal Igneous Injection. 201 



Completely fluid gabbroid magma would therefore not be 

 expected at a depth short of 21 miles even if the temperature 

 gradient were truly rectilinear. Allowing an extra mile of 

 depth to correspond to a possible slight weakening of the 

 gradient at these depths, it follows that a shell of gabbro at 

 the depth of 25 miles might be completely fluid. The enor- 

 mous predominance of basalt in the world's lava-fields, especially 

 among the lava-floods of fissure eruptions, suggests the strong 

 probability that the fluid material immediately beneath the 

 crust has actually the composition of gabbro. It thus appears 

 permissible to regard 25 miles as representing nearly the thick- 

 ness of the crust. 



The strong compression of the substratum would decrease 

 the average intermolecular distance and thus increase the vis- 

 cosity, but the material remains a true liquid. Though the 

 viscosity of such a liquid is high, yet the possibility of its 

 infinite deformation and its capacity of transmitting pressures 

 hydrostatically are, in the long periods of geological time, as 

 perfect as if the substratum were comparable to water in 

 fluidity. Its material is a true fluid and not a solid either 

 rigid or plastic. If, however, the substratum material, still 

 preserving its high temperature, is injected into higher levels 

 in the crust, where pressures are less, it may become highly 

 mobile under small stresses. " It is important to remember, 

 too, that the very act of flow of a viscous fluid, by the defini- 

 tion of viscosity, produces internal friction and additional heat 

 and renders it more fluid."* 



Compression of the substratum • flotation of the crust. — 

 The gabbroid substratum at its upper surface bears a pressure 

 of about 12,000 atmospheres. This vast pressure must com- 

 press magma very considerably. Barus has demonstrated that 

 liquid napthalene is about four times more compressible than 

 solid napthalene at the same temperature. f If gabbro obeys 

 the same law even approximately under the assumed conditions 

 of pressure and temperature, it is possible to estimate roughly 

 the density of the uppermost shell of the substratum. Let it 

 be assumed, for example, that the specific gravity of crystal- 

 lized gabbro at 0° C is 3'02 and the volume 100*0. Its dens- 

 ity when/completely molten at 1250° C. and at 1 atm. would be 

 about 2*54; the volume would be 118*84 Its compressibility 

 would be about four times that of solid gabbro, which is prob- 

 ably of about the same compressibility as that of glass, viz., 

 nearly '0000026 per atmosphere. The pressure of 12,000 at- 

 mospheres would reduce the volume to 106'1; the density 



* Quoted from a letter to the writer from Dr. Lane. 

 |C. Barus. Bull. 96, U. S. Geol. Surv., p. 83 ff., 1892. 

 tC. Barus, Bull. 103, U. S. Geol. Surv., p. 25 ff., 1893. 



