R. A. Daly- — Mechanics of Igneous Intrusion. 125 



of high viscosity in the magma in its latest phase of intrusion. 

 By reason of the enormous pressures in depth, this strong 

 viscosity would not prevent the injection of the magma into 

 the shattered zone. On the other hand, the long, narrow 

 apophyses running out to great distances from the tonalite- 

 granite mass must have been formed in the longer period of 

 high fluidity. To that period of maximum shattering, rifting 

 and stoping, the opening of the magma-chamber is to be referred. 

 Since the specific gravity of the younger granite averages 

 but 2*61, and since the stocks of that intrusive have invaded 

 formations practically identical with those displaced by the 

 tonalite-granite, the foregoing argument applies with equal or 

 greater force to the later intrusions. The shattering is again 

 found about these stocks, but it is much less impressive than it 

 is along the S.E. contact of the batholith tongue — perhaps 

 because of the smaller size and more effective stoping power 

 of the alkaline granite bodies. 



Stability of the Roofs of Magma Chambers. 



The problem remains as to how far in a vertical direction 

 such stoping has gone in the case, for example, of such an 

 immense batholith as that represented in the Nelson granite. 

 Did the destructive action go on until the magma had worked 

 its way to the earth's surface 1 Was there at any time an 

 extensive foundering of the thinned crust overlying the bath- 

 olith? Is plutonic energy so nicely balanced with, or con- 

 trolled in its exhibition by, the enormous amount of work to 

 be performed in opening a batholith chamber that the stability 

 of its vault is never endangered by prolonged stoping? There 

 seems to be no evidence of such foundering and world-shaking 

 catastrophe in the later geological ages or even in Paleozoic 

 times. According to Kelvin's classic speculation there has 

 occurred a great break in the history of the earth. That break 

 was coincident with the final encrustation of the planet as it 

 cooled from a molten state — "the date of the first establish- 

 ment of that consistentior status, which, according to Leibnitz, 

 is the initial date of all geological history."* Kelvin has con- 

 cluded that previous to final, complete encrustation, the earth 

 attained its present high rigidity by the submergence of the 

 partial crusts produced by the freezing of a less dense fluid 

 mass. The close knitting together of these foundered crust- 

 blocks thus afforded a pre-Paleozoic earth so far cooled down, 

 so strongly bound in its continuous crust, as not to suffer fur- 

 ther from the catastrophic violence of wholesale crustal found- 

 ering. The speculation leads directly to the further query 

 whether the peculiar structural complexity of the Archean 



* Lord Kelvin, Math, and Phys. Papers, London, 1890, vol. iii, p. 297. 



