Van Hise — MetarnorpMsm of Rocks and Hock Flo wage. 81 



merit of oxygen and sulphur. In the upper zone oxygen re- 

 places sulphur, and at the same time may largely oxidize that 

 element. This results in great liberation of heat, and in some 

 cases also in expansion of the volume of the solid compound, 

 as in the case of the production of limonite. Oxidation may 

 take place without replacing another element, as when iron 

 protoxide is changed into sesquioxide with expansion of vol- 

 ume and liberation of heat. In the lower zone sulphur may 

 replace oxygen with condensation and with great absorption of 

 heat. 



Another set of reactions of the most fundamental impor- 

 tance and widespread character, in which the first part of van't 

 HofFs law of chemical reactions and the law of pressure stand 

 opposed to each other, and which occur in an opposite sense of 

 the two physico-chemical zones, is the mutual replacement of 

 carbon dioxide and silicon dioxide. Xear the surface carbon 

 dioxide replaces silicon dioxide, with great development of 

 heat, and expansion, provided the freed silica separates as 

 quartz. The general fact of the carbonation of the silicates 

 under these conditions the world over is well known. 



In the lower physico-chemical zone, and especially under 

 mass dynamic conditions, silica replaces carbon dioxide upon 

 the most extensive scale with great absorption of heat and 

 with condensation, comparing the combined volumes of the 

 original carbonate and silica with that of the resultant silicate. 

 As illustrations of this may be mentioned the formation of 

 woliastonite from pure limestone, of tremolite from dolomitic 

 limestone, and of actinolite and griinerite from ankerite or from 

 siderite. In the impure limestones under deep-seated con- 

 ditions, where numerous bases are present, various complicated 

 silicates form, such as other pyroxenes and am pinholes, and 

 tourmaline, chondrodite, et cetera. As illustrating the very con- 

 siderable condensation in silication woliastonite may be taken, 

 the volume of which is 31*5 per cent less than that of the com- 

 bined volumes of the calcite and silica from which it may be 

 produced. 



As a corollary to the foregoing pages is the conclusion that 

 in the upper zone, where pressure is relatively unimportant, 

 upon the average, alterations result in the expansion of the 

 volume of the rocks ; and in the deeper-seated zone, where 

 pressure is important or dominant, upon the average, the alter- 

 ations result in the contraction of the volume of the rocks. 

 It follows as a further conclusion from this that the tendency 

 of the alterations in the first zone is, upon the average, to pro- 

 duce minerals of lower specific gravity than the original min- 

 erals, while in the deeper-seated zone the tendency upon the 

 average is to produce minerals of higher specific gravity. 



Am. Jour. Sci.— Fourth Series, Vol. VI, No. 31.— July, 1898. 

 6 



