768 PRINCIPLES OF STRATIGRAPHY 



carbonate, which is abundant at times. Chlorinization results in the 

 formation of NaCl. KCl, NH.Cl, FeCl,, CuCl,, MnCl, and. other 

 chlorides. By the action of sulphuric acid alums are formed, of 

 which the potash and soda alums are the most abundant. Gypsum 

 is formed by the action of the calcium-bearing compounds and sul- 

 phuric acid, this latter also forming Glauber's salts, sodium sul- 

 phate and potassium sulphate. The hydrosulphuric acid acting upon 

 various compounds forms sulphides, and so in turn each acid acting 

 upon the rocks forms compounds of various kinds. 



These activities are of course not always sharply isolated. In- 

 deed, solfataric and fumarolic actions are a common accompaniment 

 of igneous activity and thus the alteration of the rocks is a com- 

 plex, of pyro-, hydro- and atmometamorphism. 



4. Biometamorphism. A change in the rock due to contact with 

 organisms, or biometamorphism, is of little effect, when we remem- 

 ber that the change must be produced by the physiological activities 

 of the organism. Thus the changing of clay into bricks by baking 

 is not primarily an organic process, though directed by man. It is 

 a case of pyrometamorphism, even though the heat is artificially 

 supplied. Disintegration of rocks by growing organisms is perhaps 

 their only significant metamorphic activity. 



DYNAMIC OR PRESSURE METAMORPHISM, OR 

 SYMPHRATTISM. 



When rocks are subjected to orogenic disturbances, their internal 

 structure will be affected by the movement and pressure, and to 

 some extent by the heat developed by these processes. The effect 

 in slight deformations is often confined to the planes of gliding 

 along which deformation has taken place. Here smooth gliding sur- 

 faces or slickensidcs are formed, often with the development of a 

 thin layer of mineral matter, and marked by striations and flutings 

 which indicate the direction of motion. Extreme polishing some- 

 times results from such movements. Here belong the glacial stria- 

 tions and groovings which are slickensidcs on a large scale. 



The development of a mineral coating on the polished surface 

 of the gliding plain may be the direct result of the energy liber- 

 ated by the gliding process, or it may be the secondary deposit in 

 the fissure, when the surface of the mineral will take on the cast 

 of the striated surface. Hematite, chlorite, calcite, pyrite and other 

 minerals have been active in such wise. In some cases, however, 

 the mineral may have been deposited in the fissure before slipping 



