Invasion to Regional Metamorpliism. 179 



remains at unity, since increase of pressure tends to 

 increase its density, and increase of temperature tends 

 to decrease it. As the conditions of volume must vary 

 greatly, there is no need here of attempting refinements 

 of calculation. 



For computing the density of carbon dioxide or other 

 gases, assume that the physical conditions are those of 

 the critical pressure and temperature of water, 195 

 atmospheres and 370° C, since this pressure is reached 

 at a depth of about half a mile, and the temperature 

 is one at which hydrates retain much of their water of 

 chemical combination, even under the absence of external 

 pressures as at the surface of the earth. Under these 

 conditions, water also is a gas and incapable of con- 

 densing other gases within it. For these conditions the 

 density of carbon dioxide was computed according to 

 Van der WaaPs equation :^^ 



{l>-\-~;){v-h)=RT 



In this 



p = pressure in atmospheres = 195 



f = 1 when ^ is 1 



E-= 0.003,66 



T = absolute temperature 



a = 0.009,131 constant for carbon dioxide 



& = 0.002,427 



The density of carbon dioxide (for p = 195 and T = 

 370 + 273) is 0.172 of the density of liquid water (at 

 p=l andT = 4 + 273). 



Consider the change of biotite to serpentine. As col- 

 lateral products Van Hise gives kaolin and gibbsite 

 according to the following equation: 



6KHM^, A 1,8130., -I- 18H,0 + SCO. = 4H,M^3SiA + 

 ^ 5H,A],SiA + 2A1 (OH)^ + SK^CO^ 



It is of course possible that sericite might form to 

 some degree in place of kaolin, gibbsite, and potassium 

 carbonate. The equation will, however, illustrate the 

 problem. 



The volume equation as given by Van Hise^^ does not 

 figure in the water and carbonic acid and assumes the 

 elimination of the potassium carbonate in solution. The 



^' Walker 's Introduction to Physical Chemistry, pp. 94-97. 

 '« C. R. Van Hise, U. S. Geol. Survey, Mon. 47, p. 378, 1904. 



