GNEISS AND LIMESTONE CONTACT PHENOMENA 281 



These brief statements are given here in order to call the 

 petrologist's attention to these relations, formerly very little studied. 



VARIATION OF MINERAL PARAGENESIS IN METAMORPHIC LIMESTONE 



Sedimentary limestones generally consist of carbonates of 

 calcium and magnesium, and of silica, in the form of quartz. Very 

 often they also bear clayey materials, containing alumina, iron 

 oxides, potash, etc. When such material is brought from surface 

 conditions down to deeper levels where a higher temperature 

 prevails there will occur reactions of the general type: 



RC03+SiO,= RSiOj+CO^. 



The temperature limit above which the right-hand side of the 

 foregoing equation represents the stable combination, or the 

 transformation point of the chemical system, is raised with pressure. 

 For the reaction CaC03+Si02 = CaSi03 + C02, V. M. Goldschmidt 

 calculated the approximate equilibrium curve on the basis of 

 Nernst's affinity theorem.^ According to this curve, the equilib- 

 rium temperature increases rapidly with the pressure, being 850° 

 at 300 atm. Thereafter the rise of the equilibrium temperature 

 with pressure should be nearly linear and so slow that as much as 

 15,000 atm. would be needed at 950°. Considering rocks of 

 neighboring occurrences which have originated under similar 

 pressures but at different temperatures, as is often the case at 

 the contacts of igneous masses, the limestone indicates what parts 

 of it have been heated above the reaction point. If the pressure 

 be known, the temperature may be estimated in degrees. In the 

 case of rocks whose metamorphism has taken place under pressures 

 of more than 3,000 atm., or at depths of more than 10 kilometers, 

 the equilibrium temperature is but slightly variable with pressure, 

 and the mineral composition of silica-bearing limestones is mainly 

 an indicator of temperature, or the limestone may be used as a 

 geologic thermometer. 



This curve is not claimed to be more than approximate. Experi- 

 mental investigation may give considerably different results. 



'V. M. Goldschmidt, "Die Gesetze der Gesteinsmetamorphose," Vid. selsk. Skr. 

 Mat.-naturv. Kl., No. 22 (1912). 



