METAMORPHOSED PSEPHITES. 857 



cementation. But it should be stated that the psephites are the most 

 favorable of all mechanical deposits for alteration below the water. This 

 follows, first, from their coarseness, which is ordinarily such as to furnish 

 openings of supercapillary size in which the resistance to water circulation 

 is comparatively small; and, second, from the great variety of readily 

 alterable minerals which they contain The alteration of these minerals by 

 oxidation, hydration, and carbonation, resulting in increase of volume, 

 might supply considerable quantities of material to cement the interstices 

 of the rocks without additions of any material from an outside source, and 

 therefore not require vigorous circulation for its accomplishment. But, as 

 already stated, it is believed that the main work of induration is accom- 

 plished after the rocks have emerged from the sea and erosion has begun. 

 After arising from the sea, with differences of elevation, a vigorous under- 

 ground circulation is set up; material is steadily contributed from the belt 

 of weathering; consequently the processes of cementation and metasomatism 

 take place with comparative rapidity, and thus transform the unconsolidated 

 pebbles, gravels, and bowlders to conglomerates. 



Where indurated conglomerates pass into the zone of anamorphism 

 they may be further altered by the reactions of that zone. Where the 

 conditions are mass-static the alterations do not obliterate previous textures 

 and structures, and therefore the rocks remain conglomerates. But recrys- 

 tallization of matrix and pebbles alike may take place on an extensive scale 

 by silication and dehydration, with the production of the heavy anhydrous 

 minerals characteristic of the zone of anamorphism. 



SCHIST-CONGLOMERATE AND GNEISS-PSEPHITE, OR CONGLOMERATE-SCHIST AND PSEPHITE-GNEISS. 



Where any of the previously described psephites — i. e., pebble, gravel," 

 and bowlder deposits, or conglomerates — pass into the zone of anamor- 

 phism and are there subjected to mass-mechanical action, recrystallization 

 and granulation, and the development of a schistose structure, take place 

 under the principles described in Chapter VIII. (See pp. 685-696.) Because 

 of the great variety of minerals which may be joresent in the psephites, all 

 the minerals which develop in the lower zone under mass-dynamic action 

 may form, and they may be most intimately intermingled. In general, the 

 process of recrystallization and granulation is complete for the matrix at a 

 stage when the pebbles are still very distinct, although, of course, in such 



