746 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



been of a low order when many of these persistent flows were erupted. Nothing 

 can seem more probable than that the relatively small fall in temperature, 

 represented in the passage of a thinly molten magma to a toughly viscous con- 

 dition, has actually taken place in plutonic bodies. Doelter has shown experi- 

 mentally that that decline in temperature under surface conditions may be 

 from 1240° C. to 1150° C. for granite, from 1070° to 1010° C. for phonolite, 

 and from 1060° to 992° C. for basalt. The presence of water and other mineral- 

 izers in granitic magmas must add to their mobility, as held by many writers 

 including Brogger, whose general argument for liquidity seems irrefutable.* 



Even granting that the kinetic viscosity of a plutonic magma is thousands 

 of times that of water, 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 Ladenburg' has lately shown that small steel spheres will, 

 in a few minutes, sink through twenty centimetres of Venetian turpentine, a sub- 

 stance 10(\,000 times as viscous as water.:}: Ladenburg's experiments have veri- 

 fied the generally accepted equation expressing the rate of sinking of a sphere 

 in a strongly viscous fluid: 



2 gr*(d-d') 



x = -- 



9 v 



where x = the velocity of the sphere when the motion is steady ; g = the accelera- 

 tion of gravity; <i = the density of the sphere; d' = 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. 



Doelter estimates that the pressure of from 7,500 to 11,000 metres 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. G. F. 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 kilometres underground may, then, be possibly no 

 more than a few thousand times that of water at the earth's surface. One must 



* W. C. Brogger, Die Eruptivgesteine des Kristianagebietes, Vol. 3, 1898, p. 336. 



t 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. 



X Annalen der Physik, Vol. 22, 1907, p. 287. 



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

 1902, p. 222. 



** C. Doelter, Physikalisch-chemische Mineralogie, Leipzig, 1905, p. 110. 



