200 JR. A. Daly — Abyssal Igneous Injection. 



be made to the actual facts of outdoor uatore which would be 

 essential to a thorough presentation of the hypothesis, but it is 

 true that each argument has been made, as far as possible to 

 the writer, with attention to observations in the field. In 

 fact, it has been the direct call for some kind of explanation 

 of structures and rock-associations in the British Columbia 

 mountains that has prompted this hypothesis. The principle 

 of advancing reasonable deduction beyond the region of 

 observed fact needs no apology. Nowhere in dynamic geol- 

 ogy is an appeal to the realm of the unseen more necessary 

 than in the problems of orogeny and igneous intrusion. These 

 problems have already reached the stage where more observa- 

 tions on the visible part of the earth-crust are not so funda- 

 mentally necessary as the reference of the abundant, now 

 accessible, and well recorded observations to an intelligent 

 imagining of the gigantic forces and processes resident in the 

 earth's invisible interior. The orology of the future, even 

 more than that of the past, must rest on well-regulated specu- 

 lation. 



The hypothesis is phrased in terms of a solid crust floating 

 on a liquid substratum. It is possible that gases originally 

 absorbed in the substratum, according to the conception of 

 Lane,* should also be considered ; but, on a following page, a 

 reason for excluding them from any large share in crustal 

 deformation is briefly noted. On that ground and because of 

 the relative simplicity secured for the hypothesis as set forth, 

 the dynamic influence of the absorbed gases is not discussed. 

 It will be seen, however, that the expansional energy of gases 

 possibly given off during the solidification of the substratum 

 would furnish a condition favorable to the hypothesis. 



Thickness of the Crust. — The depth of the level where the 

 fluid substratum is first encountered has been estimated in 

 several ways. Of these estimates, that founded on the most 

 reliable determinations of the temperature gradient and of the 

 relation between pressure and the fusion point, is clearly 

 preferable. Kelvin has shown that for about 25 miles of 

 depth the gradient would be essentially rectilinear in a cool- 

 ing globe. The generally accepted gradient is approximately 

 one degree Centigrade for 100 feet of depth. This gradient 

 implies that at the depth of about 22'5 miles a gabbroid magma 

 would possess the temperature appropriate to complete fusion 

 at atmospheric pressure; (diabase melts at 1170° C. and solidi- 

 fies at 1095° C. — Barns). Vogt has recently calculated that 

 the pressure of 22*5 miles of rock would raise the fusion-point 

 about 50°C.f 



*Bull. Geological Society of America, vol. v, 1894, p. 259. 

 f J. H. L. Vogt, Die Silikatschmelzlosungen, part 2, Videnskabs-Selskabets 

 Skrifter, I. Math.-naturv. Klasse, Christiania, 1901/ p. 210. 



