894 A TREATISE ON METAMORPHISM. 



cementation. This simply means that the pelites begin to pass into the 

 zone of anamorphism at less depth than do other rocks. 



Cementation is of consequence for the pelites, but not of such dominant 

 importance as in the psammites. The muds are practically impervious to 

 percolating- waters, because the openings between the particles are sub- 

 capillary. (See Chapter III, p. 143.) Hence very little material can be 

 brought in from an outside source to fill the minute interspaces of the 

 particles of mud; but by the metasomatic processes within the minute 

 mineral particles the solutions obtain material which may be deposited 

 between the grains. As the water is squeezed out by pressure, not only 

 are the particles brought closer together, but as the water moves toward 

 the place of escape the pressure lessens and material from solution is 

 precipitated, and thus cements the particles. (See p. 114.) 



The change of mud to shale is therefore largely a mechanical process 

 of consolidation, resulting in welding; but the chemical processes of meta- 

 somatism and cementation also are important. 



Where the shales pass into the zone of anamorphism and the conditions 

 are mass-static, the processes of that zone are inaugurated. Dehydration 

 may be carried far. The silication of the subordinate amouuts of carbo- 

 nates which are so generally present in the shales may take place. In the 

 shales there is very frequently organic material, and this results in deoxida- 

 tion. By metasomatism the heavy minerals develop. The rocks thus 

 altered may become compact, strong rocks, composed of anhydrous minerals. 

 The reactions for shales in the zone of anamorphism are those which pro- 

 duce the rocks next to be considered. 



SLATE-PELITE, SCHIST-PELITE, AND GNEISS-PELITE; OR PELITE-SLATE, PELITE-SCHIST, AND PELITE-GNEISS. 



Where shales are buried so deep as to pass into the zone of anamor- 

 phism and mass-mechanical action takes place, slate-pelites, schist-pelites, 

 and gneiss-pelites, or pelite-slates, pelite-schists, and pelite-gneisses are 

 produced. Where the depth is not great and the mass-mechanical action is 

 not very severe slates are likely to form. Where the depth is greater and 

 the mass-mechanical action is severe schists or gneisses are likely to 

 develop. As the shales are very weak rocks, the depth at which the 

 reactions of the zone of anamorphism are inaugurated is not gveat. It is 

 certainly less, on the average, than for any of the previous families. 



