MECHANICAL WORK. 671 



In the zone of anamorphism, in consequence of pressure in all directions 

 greater than the crushing strength of the rocks, and the resulting invariably 

 close contact of the mineral particles, the process of welding is of far greater 

 consequence than in the belt of cementation, although it has been shown 

 that in the latter belt welding does take place under favorable conditions. 

 (See pp. 595-597.) In short, while welding is an exceptional process in the 

 zone of katamorphism, it is a universal process for the zone of anamorphism. 



The depth at which welding occurs is different for different substances. 

 Such plastic substances as coal and clay may be welded at very moderate 

 depth, while the strong and refractory rocks, such as quartzites, require a 

 very considerable depth. 



STRAIN WITHIN ELASTIC LIMIT. 



Within the zone of anamorphism all the mineral particles of all rocks 

 are in a high state of strain at all times. Where the conditions have been 

 quiescent for a long time and the mineral particles have readjusted them- 

 selves to the conditions, it may be supposed that the stresses are approxi- 

 mately equal in all directions, and therefore that the conditions approach 

 those of hydrostatics. Under such circumstances the pressure would be 

 the same in all directions within a mineral particle, and the only tendency 

 would be to elastically condense it. The strain would be one of simple 

 compression; or putting it in physical terms, negative homogeneous dilata- 

 tion. 01 The molecules of a given mineral particle, as, for instance, one of 

 quartz, are pressed together by virtue of the elasticity of the mineral, and a 

 given particle occupies less space than under surface conditions, where the 

 pressure is slight, Where the hydrostatic pressure is very great, recrystal- 

 lization may follow, by means of which the minerals pass into more con- 

 densed molecules, as is fully explained under " Metasomatism," and 

 especially in connection with the development of porphyritic crystals. (See 

 pp. 699-705.) 



Strain within the elastic limit in which the pressure is equal in all 

 directions is very exceptional, if it ever exists. Almost invariably the 

 pressure is unequal iu different • directions. The difference in the amount 



"Hoskins, L. M., Flow and fracture of rocks as related to structure, appendix to Van Hise, 0. R., 

 Principles of jSforth American pre-Oambrian geology: Sixteenth. Ann. Kept. U. S. Geol. Survey, pt. 1, 

 1896, p. 860. 



