REPORT OF TEE CHIEF ASTRONOMER 745 



SESSIONAL PAPER No. 25a 



roborated by the proved, similarity of silicates and carbon compounds, in (a) . 

 the linear relation of expansion to increment of temperature in the solid form 

 of each substance, in (b) the linear relation of expansion to increment of tem- 

 perature in the liquid form of each substance, and in (c) the sudden leap in 

 volumetric increment in the act of melting at any temperature. 



Barus further indicates that solid naphthalene is comparable in compres- 

 sibility with the liquid form of the same substance.* His fusion curves show 

 that, for the same increase of pressure, liquid naphthalene gains in specific 

 gravity about, twice as fast as solid naphthalene. The compressibility of a 

 fused silicate rock is perhaps, then, twice that of the same rock when solid. 

 But his diabase fusion curve demonstrates that the thermal expansibility of 

 the liquid rock is about 1-9 times as great as that of the solid rock. Thus a 

 block of cold, solid gabbro immersed in a deep-seated molten magma of the same 

 chemical composition, would be less condensed by the pressure than the molten 

 rock, but the effect on relative densities would be partly compensated by any 

 superheating of the magma. Moreover, the compressibility of glass and of 

 crystalline silicates is known to be very low. The compression suffered by glass, 

 for example, is about -0000026 of its volume for 1 atm. The weight of even 

 10,000 metres of rock with an average density of 2-75 would cause a density 

 increase of much less than one per cent in glass. It is therefore probable that 

 the difference of density between magma and immersed block would not be 

 affected through pressure, at the great depth of 10 kilometres, by as much as 

 1 per cent of the density of either one. 



Sinking of the Shattered Blocks. — It appears from Table LI. and LII. that 

 nearly all xenoliths must sink in any molten granite or syenite; most xenoliths 

 must sink in molten quartz-diorite, tonalite or acid gabbro. Many xenoliths 

 might float on basic gabbro but the heavier schists and gneisses must sink in 

 even very dense gabbro magmas at 1300° C. 



Giving, then, the highest permissible values to the specific gravities of mag- 

 mas, it is still true that blocks, such as are shattered from the wall or roof of 

 a batholith, must sink when immersed in most magmas at atmospheric pressure. 



It has been objected to the stoping hypothesis that the viscosity of granitic 

 magmas is too great to allow of the sinking of blocks even much denser than 

 those magmas. This objection has, however, never been sustained by definite 

 experimental or field proofs. The xenoliths visible along batholithic contacts 

 have assuredly not sunk far from their former positions in wall or roof and the 

 reason for this must be sought in the high viscosity of the magma. High 

 viscosity is an essential attribute of a nearly frozen magma. The phenomena of 

 fractional crystallization and of magmatic differentiation unquestionably show 

 that each 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 



* Amer. Jour. Science, Vol. 42, 1891, p. 140. 



