JR. A. Daly — Mechanics of Igneous Intrusion. 29 



plutonic magma must pass through a long period of mobility. 

 The most viscous of granitic magmas, the rhyolitic, issues at 

 the earth's surface with such fluidity that the rhyolite often 

 covers many square miles with a single thin sheet. The 

 absolute viscosity of the Yellowstone Park rhyolites must 

 have been of a low order when many of these persistent flows 

 were erupted.* 



Even granting that the kinetic viscosity of a plutonic 

 magma is thousands of times that of water, it seems inevitable 

 that it could not support xenoliths more dense than itself. 

 In a few days or weeks stones will sink through, and corks 

 will rise through, a mass of pitch, the viscosity of which is 

 more than a million of millions of times that of water, f 

 Laden burg has lately shown that small steel spheres will, in a 

 few minutes, sink through twenty centimeters of Venetian 

 turpentine, a substance 100,000 times as viscous as water.f 

 Laclenburg's experiments have verified the generally accepted 

 equation expressing the rate of sinking of a sphere in a strongly 

 viscous fluid : 



2 qr 2 (d - d') 



9 v 



were x — the velocity of the sphere when the motion is steady 

 g ~ the acceleration of gravity ; d = the density of the sphere 

 cV = the density of the fluid ; r — the radius of the sphere 

 and v — the viscosity of the fluid. § The equation shows that 

 the velocity of sinking varies directly as the square of the 

 radius of the sphere. This fact may be correlated with the 

 observation so often to be made on granite contacts, that large 

 xenoliths are rare. This apparently means that, at the end of 

 the shatter-period, the viscosity is truly so high as to allow 

 of the smaller blocks being trapped at high levels in the 

 freezing magma, while the large blocks, with greater velocity, 

 shall have sunk into the depths. 



*See Atlas accompanying Monograph 32 of the IT. S. Geol. Survey. 

 King described the great rhyolite flows of Nevada as bearing " abundant 

 evidence of true fluidity at the period of ejection." IT. S. Geol. Explor. 

 40th Parallel. Sys. Geol. 1878, p. 616. 



Doelter has studied the behavior of a large number of crystalline rocks 

 and minerals during fusion. His results show that the temperature-inter- 

 val between the stage of softening and that of notable fluidity averages, for 

 the basic rocks, about 50° C, and for the acid rocks, about 90° C. (Tscher. 

 Min. u. Petrogr. Mitth. xx, p. 210, 1901.) The interval is not great and it 

 certainly seems unsafe to deny that even the most viscous, because cooled, 

 lavas were fluid in depth. 



f Jamin et Bouty, Cours de Physique, tome I, 2e fascicule, Paris 1888, 

 p. 135 ; cf. Daniell's Text-book of the Principles of Physics, 2d ed., London, 

 1885, p. 211. 



% Annalen der Physik, xxii, p. 287, 1907. 



§Poynting and Thomson, Text-book of Physics, Properties of Matter. 

 London, p. 222, 1902. 



