Invasion to Regional Metamorpliism. 181 



The question is logically raised, why, after the gas 

 pressure has fallen, does not dehydration proceed so as to 

 nullify the previous action! To some degree it may, 

 but a mineral species can exist in a wider range of envi- 

 ronment than that which determined its production. The 

 chlorite schists are especially associated with regional 

 mashing and this was without doubt a factor in their 

 genesis. Furthermore, although the excess of gases dis- 

 appears, the non-porous nature of the rocks does not 

 favor further elimination. 



ShjICation in Eelation to Igneous Intrusion. 



Geologic observation shows that blocks of marble may 

 be immersed in abyssal magmas and not suffer dissocia- 

 tion of the carbonic acid at the magmatic temperatures 

 and pressures. Such alterations to silicates as occur 

 are the result of reaction with the silica and other oxides 

 which are present, either in the original sediment or 

 carried in from the magma. The experimental work that 

 has been done on the relations of temperature and pres- 

 sure to the dissociation of calcium carbonate has been 

 reviewed by Konigsberger,^^ and on account of the wide 

 range of pressures represented in the experimental data, 

 he selects the work of Riesenfeld^^ for extrapolation; 

 there is, however, a considerable disagreement of 

 authorities, and the figures may be only approximate. 

 The table is of sufficient interest to be given here. 



Temperature 



Pressure 



Centigrade 



Atmospheres 



910 



1 



1000 



4 



1100 



20 



1200 



170 



1300 



2,600 



1400 



80,000 



The dissociation curve supplements the evidence of 

 wollastonite as a contact mineral which cannot exist 

 above 1150° C. in showing that the inclusions of marble 

 have never been heated to a temperature as high as 

 1300^ C, since this would require a depth to prevent 

 dissociation greater than that given by the geologic 

 evidence. 



^"^ J. Konigsberger, N. Jahrb. f . Min., etc., Beilage Bd. 32, p. 124, 1911. 

 -° E. H. Eiesenfeld, Jour. Chim. Phys., 7, 561, 1909. 



