J. P. Iddings — Origin of Quartz in Basalt. 215 



in the chemical composition of the magma is a generally 

 accepted one. What some of these changes of condition prob- 

 ably were may be indicated by the following considerations. 



Consideration of possible changes of physical condition. — 

 If we conceive of a molten, viscous magma situated at a great 

 depth beueath the earth's surface, the two factors which at 

 first appear to have the greatest influence on its existence as a 

 plastic or molten magma are temperature and pressure. 



Influence of temperature. — Other things remaining the 

 same, the magma will be more plastic the higher the tempera- 

 ture, and with a decrease of temperature below a certain point 

 consolidation will take place. ■ 



Consolidation under these circumstances would undoubtedly 

 produce crystallization, as a rapid chilling at great depths 

 within the earth is scarcely conceivable. However the nature 

 of the crystallization would vary within certain limits with the 

 rate of cooling. 



Influence of pressure. — If, now, the temperature remains 

 the same and the pressure varies, assuming that an increase of 

 pressure diminishes the mobility of the molecules of the 

 magma by condensing them, which from a theoretical stand- 

 point appears the most rational supposition, and which has re- 

 ceived support from the recent experiments of Dr. Wm. 

 Hallock ;* and it having been demonstrated that the silicate 

 mineralsf and glasses;}; belong to that class of substances which 

 contract upon solidification, we should expect that an increase 

 of pressure alone would tend to consolidate the mass. This 

 is the conclusion reached by Reyer in his work entitled, " Bei- 

 trag zur Fysik der Eruptionen und der Eruptiv-G-esteine." 

 Vienna, 1877, p. 119. 



Increased viscosity . — Dr. Hallock' s observations show that 

 cubical, statical pressure, unaccompanied by heat, simply in- 

 creases rigidity or viscosity. But that in cases where pressure 

 is allowed to produce motion by the crushing or yielding of 

 the molecules of a substance, the internal friction may gene- 

 rate heat enough to fuse the substance, that is, to reduce its 

 rigidity or viscosity. 



The solidification of a glassy, amorphous mass by increasing 

 its viscosity might convert it into a solid glass or into a crys- 

 talline mass, according to whether the viscosity was suddenly 

 increased or slowly increased. The changes being analogous 

 to the rapid increase of viscosity by chilling, or the slower 

 increase by gradual cooling, the latter allowing the molecules 

 to arrange themselves in obedience to certain chemical affini- 



* This Journal,. October. 1887. 



f Roth, Allg. u. Chem. Geol., Bd. IT, p. 52. 



JLagorio. T. M. P. M., vol. viii, 1887, 510. 



