REPORT OF THE CHIEF ASTRONOMER 573 



SESSIONAL PAPER No. 25a 



2. Orogenic axes are generally parallel to the respective axes of the gene- 

 tically connected geosynclinal prisms. 



3. Each geosynclinal prism hears contemporaneous lavas, usually basaltic 

 or andesitic. This rule is so persistent, in the Cordillera and elsewhere in the 

 world, that we may believe there is some genetic relation between the down- 

 warping and movements of the magmatic stratum beneath the earth's crust. 



4. Granitic intrusion of the batholithic order, to observed levels, always 

 follows periods of the more intense orogenic movement. This implies that the 

 greatest abyssal injections of the earth's crust by magma are genetically asso- 

 ciated with the horizontal shearing of a superficial earth-shell which is much 

 thinner than the whole crust. 



5. The topographic and structural effects of mountain-building at the 

 Eorty-ninth Parallel are clearly related to the degree of lithifaction (static 

 metamorphism) undergone by the geosynclinal sediments. A leading illustra- 

 tion is seen in the prevalence of overthrusts, horizontal shifts, and fault-blocks 

 in the strong Beltian-Cambrian terrane of the Eastern Belt, while close folding 

 and mashing are features of the less consolidated, thotigh otherwise petro- 

 graphically similar rocks of post-Mississippian age in the Western Belt. 



6. Yet, part of this contrast between the two belts is to be referred to the 

 fact that orogenic pressure has been applied with greatest intensity on the 

 Pacific (ocean) side of the Cordillera. 



Toward the close of field work on the Boundary section, the writer 

 attempted to relate these field conclusions to the prevailing contraction theory. 

 The result was to hazard a speculation on ' Abyssal Igneous Injection as a 

 Causal Condition and as an Effect of Mountain-building.'* The original sum- 

 mary of that paper may here be quoted. 



The assumptions on which the hypothesis have been based are the follow- 

 ing: — 



(a) A cooling earth superficially composed of a relatively thin crust over- 

 lying a fluid gabbro (basaltic) substratum of unknown thickness. 



(b) The substratum so much compressed by the weight of the crust as to 

 be probably able to float the crust. 



(c) Through differential cooling contraction the development of a level 

 of no strain in the crust probably not much more than six miles below the 

 earth's surface. 



(d) The accumulation of pressure in the shell of compression and the 

 simultaneous accunrulation of cooling cracks and of some of the powerful 

 tension unrelieved in the sbell below the level of zero strain. 



(e) A steady or recurrent dislocation in the shell of tension permitting 

 of the forceful injection of the fluid substratum, to which even the viscous 

 layer of the shell acts as a relatively solid mass at the moment of dislocation. 

 This dislocation has been referred to the tidal torsion of the earth's crust, but 

 subequatorial torsion on the tetrahedral theory of the earth, or crustal deforrna- 



*E. A. Daly, Amer. Jour. Science, Vol. 22, 1906, p. 195. 



