246 A TREATISE ON METAMORPHISM. 



seems probable that heat is liberated, for the transformation of aragonite to 

 calcite occurs in both the physical-chemical zones, and I know of no excep- 

 tion to the principle that such reactions take place under the first part of 

 van't Hoff's law (see pp. 107, 181). 



The change from aragonite to calcite is so complete in rocks of mod- 

 erate age that the presence of aragonite in the metamorphosed rocks is 

 almost unknown. The alteration of aragonite to calcite in both zones is of 

 considerable interest, as it presents a somewhat exceptional case. As 

 explained on pages 182-186, the common rule of change in the zone of 

 anamorphism is increase in specific gravity and increase of symmetry, 

 provided the volume change demanded will allow this. However, the 

 change of specific gravity in this case is a decrease rather than an increase, 

 and hence aragonite conforms only to the second of these rules — the first, 

 and usually the controlling rule, for the zone of anamorphism being 

 violated. These facts suggest the conclusion that in this instance sym- 

 metry is a more important factor than density — a very exceptional thing. 

 If this be so, the conclusion would follow that the symmetrical arrange- 

 ment of the molecules in calcite are those which best resist the changing 

 conditions of mass-static and mass-mechanical action in the lower zone. 

 The suggestion occurs to one that, if rocks were very deeply buried, so as 

 to be extraordinarily deep in the lithosphere, pressure might control the 

 form, and calcite alter to aragonite. This, however, is a speculation which 

 has no verification. 



THE SILICATES. 



The silicates are the most important of rock-making constituents. 

 They include natural glass and many mineral groups. The groups of 

 rock-making silicates are as follows: Feldspar, leucite, pyroxene, amphi- 

 bole, nephelite, sodalite, garnet, chrysolite, scapolite, zircon, aluminum- 

 silicate, epidote, humite, zeolite, mica, clintonite, chlorite, serpentine-talc, 

 and kaolin. Besides the members of the above groups are a number of 

 important rock-making silicates not so included. 



GLASS. 



Glass, while not a definite silicate or ordinarily included among the 

 specific minerals, is an important rock- making constituent, and therefore must 

 be treated in connection with the silicates in a treatise on metamorphism. 



