104 A TREATISE ON METAMORPHISM. 



at least twice. After one set of compounds was produced recrystallization 

 again occurred, producing heavier compounds. The first change may be 

 illustrated by the rearrangement of minerals which constitute mud so as to 

 produce mica, quartz, and feldspar, and the second stage may be illustrated 

 by the development from the latter rock of the still heavier minerals, garnet, 

 staurolite, etc. (See p. 685.) However, the process of recrystallization 

 in nature works in connection with more rapid solution of minerals where 

 strained (see pp. 95-98) and with other forces. Its full consideration is 

 therefore deferred to Chapter VIII (pp. 686-698). 



GENERAL STATEMENTS. 



Where pressure is unimportant, as near the surface of the earth, the 

 chemical reactions are ordinarily controlled by other factors than pressure; 

 but as the pressure increases, due to depth below the surface or other causes, 

 it becomes a more and more important factor in the reactions which occur. 

 But it is shown in Chapter VI, on "Weathering," that pressure may be an 

 important factor in chemical reactions comparatively near the surface. This 

 is illustrated by granitic rocks in the District of Columbia, described by 

 Merrill," which when brought to the surface underwent rapid disintegration, 

 hydration, and expansion. The pressure of a few feet of rock was appa- 

 rently sufficient to prevent the completion of these reactions, and thus it is 

 clear that the adjustment between chemical reaction and pressure may be 

 very delicate. 



However, as explained in Chapter VI, chemical reactions near the sur- 

 face do extensively take place with expansion of volume, and therefore in 

 spite of some pressure. But it is also shown (Chapter VIII) that the 

 pressure becomes a more and more potent factor in controlling the reac- 

 tions; and, finally, that there exists a lower zone of anamorphism in which 

 this is the dominant force. In this zone the chemical changes so take place 

 as to lessen the volume of the compounds, and therefore to produce heavy 

 minerals. Moreover, the reactions which occur in this lower zone are 

 frequently just the reverse of those which take place in the upper zone, 

 where pressure is unable to control, or the reactions in the two zones 

 reverse each other. 



a Merrill, G. P., Disintegration of the granitic rocks of the District of Columbia: Bull. Geol. Soc. 

 America, vol. 6, 1895, pp. 322-332. % 



