REPORT OF THE CHIEF ASTRONOMER. 575 



SESSIONAL PAPER No. 25a 



The second part of postulate d is, perhaps, the most doubtful point in the 

 speculation. Until more is known of the behaviour of silicate rocks at very- 

 high pressures, it is impossible to say how much cooling tension is left unre- 

 lieved by ' compressive extension ' at the end of the period in which conditions 

 have been ripened for mountain-building. However this may be, we must 

 believe in abyssal injection as a fact. The concentration of igneous eruptions 

 in special belts on the earth is also an evident fact. The view that repeated 

 abyssal injections within such a belt must lead to a down-warping of the surface 

 is a reasonable deduction, if the earth is cooling. 



The speculative element enters when a downwarp of geosynclinal propor- 

 tions is considered as an effect of this magmatic movement within and beneath 

 the earth's crust. So far as it goes, the observation that vulcanism is always 

 or almost always contemporary with geosynclinal sedimentation, tends to 

 strengthen that belief. In each case the number of extrusions of lava at the 

 surface may be but a small fraction of the number of abyssal injections within 

 the limits of the geosynclinal belt. It is not an extravagant assumption to hold 

 that the total volume of magma thus transferred into and through the crust may 

 be comparable to the volume of the sedimentary prism. In spite of all the diffi- 

 culties, the most satisfactory explanation for the origin and localization of the 

 great downwarps seems to be found in movements of rock-magma under cosmical 

 stresses. If this conclusion were proved, we should have gone far towards 

 solving the orogenic problem, for the mystery of a mountain-chain lies no more 

 in the folding of strata than in the development of the preliminary stratified 

 prism. 



That strong mountain-building disturbs the equilibrium of the magmatic 

 substratum and initiates granitic intrusion is already sufficiently clear. In 

 the following theoretical chapters on igneous rocks, evidence will be found for 

 the belief that visible batholiths are the roof portions of huge basaltic injections 

 chemically modified by assimilation of the injected formations. That belief 

 rests on much more than mere speculation. The question arises as to whether 

 these large-scale injections have also been facilitated by the condensation of the 

 shell of tension. The answer given in the special paper is designedly speculative 

 and the postulated mechanics may be faulty, but its central theme is strong; 

 namely, that at its closing stage as well as in the formation of the preliminary 

 geosynclinal, an orogenic movement is closely associated with the intrusion of 

 magma into the earth's crust. The writer is inclined to the view that this 

 association is close because it is genetic; and that investigators in orogeny have 

 hitherto given too little attention to the relation of subcrustal, magmatic 

 readjustments to mountain-building. 



