634 A TREATISE ON METAMORPHISM. 



silica from the solutions forms the abundant chalcedony and quartz of the 

 amygdules. 



It therefore appears probable that, in the alteration in the belt of 

 cementation of the porous rocks containing abundant silicates, expansion 

 reactions characteristic of the belt of cementation may be entirely adequate 

 not only to account for the filling of the openings, but possibly to furnish 

 additional material to the ground solutions. 



CONTRIBUTIONS FROM IGNEOUS ROCKS. 



Expansion reactions, however, are not the only cause of cementation. 

 In a subsequent section upon igneous rocks it is seen that injections locally 

 produce profound metamorphic effects. It is there explained that the chief 

 of these is the increased action of solutions due to the heat of the magma, 

 But, as is well known, magma when solidifying gives off water. This 

 water is sure to be very rich indeed in mineral content. The material thus 

 contributed to the ground water solutions is certainly a source of material 

 for cementation. This source is of importance, for volcanism is a widespread 

 geological process. So far as material of this origin is available it explains 

 the cementation of rocks notwithstanding the probability that solutions 

 emerging from the belt of cementation contain more material than when 

 entering this belt. 



SELECTIVE PRECIPITATION. 



But it is clear that the explanation thus far offered by no means fully 

 covers the matter of cementation for all rocks. Indeed, it does not fully 

 include the most important of the cases of cementation. It has been seen 

 that the dominant cement of the belt of cementation is quartz. In many 

 instances (see pp. 622, 865-866) the transformation of the great sand forma- 

 tions to sandstones and quartzites is wholly, or almost wholly, accomplished 

 by the deposition of quartz. Probably 90 per cent or more of the fractures 

 of rocks produced by deformation, such as joints, faults, bedding partings, 

 etc., are cemented by quartz. The cementation by silicates, carbonates, etc., 

 may be fully accounted for by the expansion reactions above given; but 

 what is the source of the great quantity of silica for silicification? In order 

 to answer this question we need to recall that the most distinctive process 

 of the belt of weathering is carbonation; that there, by the process of 

 carbonation of the silicates on a vast scale, colloidal silicic acid is set 



