564 A TREATISE ON METAMOKPHISM. 



but below that level the rocks are usually much less porous, although it does 

 not follow that the water circulation may not be important and rapid. Fine 

 illustrations of the sudden lessening of the pore space at water level 'are fur- 

 nished by the lead and zinc districts of the Mississippi Valley. This lessen- 

 ing of the pore space is partly due to the solution of material above the level 

 of ground water, rather than to cementation below it. But observations 

 below the level of ground water show that cementation is as certain as is 

 solution above. Sandstone formations, as is well known, are cemented 

 mainly by silica, although calcium carbonate, iron oxide, etc., are subordi- 

 nate cementing constituents. Wherever sandstones are found which have 

 long been below the level of ground water, cementation has taken place, 

 either by enlargement of the old grains or by independent deposition between 

 the grains, or by the two combined. All stages of this process are seen, 

 from those in which the grains have simply built out crystal facets that 

 sparkle in the sun, to rocks so firmly cemented as to be perfect quartzites, 

 in which fracture breaks through the cement and across the old grains 

 rather than around them." The pore spaces of arkoses, tuffs, and other 

 rocks which have long been below the level of ground water, are shown by 

 microscopical examination to have been also closed; but often the cements 

 are more variable than in the case of quartzites, frequently including 

 feldspar, pyroxenes and amphiboles, calcite, and various other minerals. 



That cementation is the universal rule for the porous rocks below the 

 level of ground water was ascertained only by microscopical study, but in 

 the field may be seen evidence of cementation of the larger fractures. The 

 filling of ancient joints, recementing the separated joint blocks, the filling 

 of fault openings, thus producing true veins, and the cementing of the frag- 

 ments of breccias are all well-known phenomena. Where rocks have been 

 broken and buried deep enough to be below the level of ground water and 

 have remained there long, it is found to be an almost universal rule that the 

 filling of these larger openings has begun; and usually, so far as the older 

 openings are concerned, the process has been practically completed, although 

 recent earth movements may have produced openings which have not been 

 closed. 



The openings which are being filled in the belt of cementation vary in 

 size from minute pores between the grains to great caves. Interesting 



o Irving, R. D., and Van Hise, C. R., On secondary enlargements of mineral fragments in certain 

 rocks: Bull. U. S. Geol. Survey No. 8, 1884, pp. 1-56. 



