30 JR. A. Daly — Mechanics of Igneous Intrusion. 



Doelter estimates that the pressure of from 7500 to 11,000 

 meters of rocks increases magmatic viscosity no more than 20 

 to 30 per cent.* If the increment be anywhere near this 

 value we may be certain that the viscosity of superheated, 

 plutonic magma is relatively low. Becker has calculated that 

 the viscosity of a Hawaiian basaltic flow, not one of the most 

 fluid, was, at eruption, about fifty times that of water. The 

 more fluid rhyolite flows may have viscosity a thousand times 

 greater than that of water. The corresponding viscosities of 

 the same magmas when ten kilometers underground may, 

 then, be possibly no more than from sixty to fifteen hundred 

 times that of water. One must conclude that a xenolith, 

 even very slightly denser than such a plutonic magma, must 

 sink into it. Since such magmas necessarily cool with extreme 

 slowness, there is evidently good ground for believing that 

 an enormous amount of solid rock could be engulfed before 

 practical rigidity is established. The average xenolith must 

 sink in a less dense magma with the viscosity of pitch — yet 

 how much more rapidly in marina possessing the low viscosity 

 which is postulated in any of the ruling theories of plu tonic- 

 rock genesis ! 



Problem of the cover. — The stoping hypothesis presents an 

 obvious principal difficulty ; it refers to the apparent danger of 

 the foundering of the roofs covering the larger batholiths. 

 Under plutonic conditions (at depths of from three to ten 

 kilometers) the average molten granite would have a specific 

 gravity no higher than 2'10. The average rock of its roof has 

 a specific gravity of about 2*70. If, then, through orogenic 

 movement, a large mass of the roof-rock became once wholly 

 immersed in the granite, it would not only founder itself but 

 through subsequent buckling the whole roof might collapse 

 and founder in sections. Such a catastrophe has almost cer- 

 tainly not happened in the case of any Paleozoic or later 

 batholithic intrusion. This difficulty has been emphasized by 

 Barrel!, who has justly given it a prominent place in his 

 monograph. f Lawson speaks of batholiths 100 miles in diam- 

 eter and also finds the necessity of explaining their roof- 

 support as a principal ground of unfavorable criticism.;}; 



The present writer cannot claim to have solved this problem, 

 but he does not find it to form a fatal objection to the hypo- 

 thesis. In the first place, it seems clear that all the other 

 hypotheses of granitic intrusion are facing the same dilemma. 

 All of them expressly or tacitly postulate some degree of 

 fluidity in each granitic mass as it either replaces or displaces 



* Physikalisch-ckemische Mineralogie, Leipzig, p. 110, 1905. 



fOp. cit., p. 172. 



% Science, xxv, p. 620, 1907. 



