RELATIONS OF DIFFUSION TO CEMENTATION. 639 



equilibrium by movement of the dissolved material iu a direction against 

 the current, the descending waters continuously take material into solution ; 

 or, if not that, at least deposit little or no material; and thus a formation 

 or an area in a formation below the level of ground water may not be 

 cemented or indurated. In this manner may be explained the streaky 

 cementation and induration of sandstones and quartzites, and very frequent 

 preponderant solution below the level of ground water where there are 

 strong descending currents (see p. 604). In places where the downward 

 movement was slow and regular, cementation and induration are in an 

 advanced stage or complete; while at places where there was rapid descend- 

 ing movement of the waters there was little deposition, and the material 

 is but feebly or not at all cemented. 



Besides the greater movements of diffusion connected with general 

 currents of ground water, diffusion is unquestionably of great importance 

 in the short movements of the ground water in the subcapillary openings 

 between the single mineral particles or aggregates of mineral particles 

 and between the adjacent capillary and supercapillary openings in which 

 the main water currents travel. In the subcapillary openings, during the 

 metasomatic processes, the water becomes saturated with material. Even 

 if the water within the subcapillary openings itself does not move, the 

 material in solution slowly migrates by diffusion toward the larger open- 

 ings containing circulating waters, and vice versa. Therefore the materials 

 of the small openings and those of the main water currents meet. The 

 latter may carry not only recently acquired material, but material obtained 

 long before, perhaps at great distances, or even from the belt of weathering 

 above. As a result of this meeting selective precipitation takes place. 



CONCLUSION. 



From the foregoing pages I conclude that cementation is caused by 

 the expansion reactions, by contributions from magmas, by selective precipi- 

 tation, and by diffusion, and that these processes are adequate to explain 

 the process, notwithstanding that the waters issuing from the belt of cemen- 

 tation contain large quantities of material in solution. In the actual precip- 

 itation in the openings of rocks these different factors work together, not 

 separately. 



