PHYSICAL-CHEMICAL FACTORS 363 



PRESSURE AND VOLUME. 



Pressure lessens the volume and therefore tends to preserve and to pro- 

 duce minerals which have a high specific gravity. Where the pressure 

 is small this factor is relatively inefficient and consequently other factors 

 usually control, and many minerals of low specific gravity form. But even 

 where the pressure is small it is not unimportant, at least retarding reactions 

 which would otherwise occur. This is illustrated by a partially altered 

 rock described by Merrill," which seemed solid when confined by the 

 surrounding rock, but which when brought to the surface from a depth of 

 a few feet, and thus freed from pressure, rapidly decomposed and disinte- 

 grated. Where the pressure is great it is likely to be a determinative 

 force, controlling the reactions. At the depths of the zone of anamorphism 

 the uniform production of anhydrous or slightly hydrous minerals of higher 

 average specific gravity than those formed in the zone of katamorphism is 

 clear evidence of the dominance of pressure. 



In this connection it will be well to mention the mineral groups, with 

 their specific gravities, which are more characteristic of the zones of 

 katamorphism and anamorphism. 



The characteristic products of the zone of katamorphism are: Of the 

 oxides, (1) those of silicon, opal, chalcedony, and quartz (sp. gr, 2.1 to 

 2.654); (2) those of iron, including the hydrous and anhydrous ferric- 

 oxides (sp. gr. 3.80 to 5.225); (3) the hydrous aluminum oxides, gibbsite 

 and diaspore (sp. gr. 2.35 and 3.40); of the carbonates, calcite and' dolo- 

 mite (sp. gr. 2.7135 and 2.85); of the silicates, (1) the epidote-zoisite group 

 (sp. gr. 3.25 to 4.20); (2) zeolite group (sp. gr. 2.04 to 2.40); (3) chlorite 

 group (sp. gr. 2.60 to 2.96); (4) serpentine-talc group (sp. gr. 2.50 to 2.80); 

 and kaolin group (sp. gr. 2.6 to 2.63). (See pp. 519-520, 621-627.) 



The characteristic important mineral groups formed in the zone of 

 anamorphism are as follows: Of the sulphides, pyrite, and pyrrhotite (sp. 

 gr. 5.025 and 4.61); of the oxides, (1) those of silicon, chert, chalcedony, 

 and quartz (sp. gr. 2.6 to 2.654); (2) those of iron, hematite, magnetite, and 

 ilmenite (sp. gr. 5.225, 5.174, and 4.75); (3) those of aluminum, corundum 

 (sp. gr. 4.025); (4) those of titanium, rutile, octahedrite, and brookite (sp. 

 gr. 4.215, 3.885, and 3.975); of the silicates, (1) the feldspar group (sp. gr. 



« Merrill, George P., Eocks, rock weathering, and soils, Macmillan Co., New York, 1897, pp. 

 252-253. 



