Barrell — Physical Effects of Contact Metamorphism. 285 



A few metamorphic minerals, such as epidote, vesuvianite 

 and biotite, may carry as much as 2 per cent of combined 

 water. "With such exceptions the water is uniformly and 

 completely expelled, and by that amount in which it was 

 contained in the original strata the mass is lessened. Carbonic 

 acid is only expelled where the siliceous impurities of the 

 limestone are sufficient to combine with the lime set free, 

 forming lime silicates. This ability of deeply-buried lime- 

 stones to retain their carbonic acid when intensely heated, if 

 free from other impurities, has been noted by a number of 

 observers, and is well shown in the Elkhorn district, Montana, 

 where marbles in some cases exist within two to four feet of 

 the granite contact, having retained their carbonic acid under 

 temperatures which at the surface would have led to its 

 immediate expulsion. In a general way the losses may be 

 tabulated as follows : 



Pure Types. Loss in Weight. 



Sandstones per cent. 



Claystones 4 to 10 " " 



Limestones " " 



Mixed Types. Loss in Weight. 



Arenaceous claystones 2 to 6 per cent. 



Arenaceous limestones 5 to 30 " " 



Argillaceous limestones 5 to 23 " " 



In addition to the above, the greater porosity of the 

 unmetamorphosed sediments enables them to retain a larger 

 amount of hydroscopic moisture, to be expelled along with 

 that in chemical combination. 



Loss of volume. — The losses in volume are still more strik- 

 ing than those of mass, and are due to three causes. First, 

 decreased porosity ; second, losses of water and carbonic acid ; 

 third, crystallization into minerals of greater density. The 

 porosity or amount of vacant space of rocks of the same 

 composition may vary within quite wide limits, sandstones, 

 running from 5 to 28 per cent, limestones 0*2 to 13 per cent, 

 shales probably from 5 to 15 per cent. The various kinds of 

 hornfels probably possess a porosity of from 0*5 to 3 per cent. 

 Taking the differences between the original and resulting 

 porosity, the losses in volume from this source, by metamor- 

 phism in a general way 3 may be set down as follows : 



Kinds of rocks. Loss in Porosity. 



Sandstone to quartzite 5 to 15 per cent 

 Argillaceous sandstone 



to hornfels 5 " " 



Impure limestones to 



hornfels 1 to 5 " " 



