670 A TREATISE ON METAMORPHISM. 



sudden, but less marked with the belt of cementation, where the variations 

 in conditions are somewhat gradual, and therefore approximate in this 

 respect those of the zone of anamorphism. 



WORK IX ZONE OF ANAMORPHISM. 



As in the zone of katamorphisnx, the work from a physical-chemical 

 point of view may be divided into mechanical work, chemical work, and 

 igneous work. 



MECHANICAL WORK. 



Mechanical work comprises welding, strain within the elastic limit, and 

 strain beyond the elastic limit. 



WELDING. 



Welding is the cohering of the rock particles in consequence of 

 pressure which brings them so close together that they are within the limit 

 of molecular attraction of one another. It has been explained that within 

 the zone of anamorphism the pressures iu all directions are greater than the 

 crushing strength of the rocks, and that if openings could be supposed to 

 be produced they would necessarily be closed because of the incapacity of 

 the rocks to support themselves. This pressure is so great that the particles 

 are brought within the limits of molecular attraction, and therefore are 

 welded for the same reason that metals weld under proper conditions. That 

 this conclusion, based upon physical reasoning, is certainly true, is shown 

 by the invariably strong cohesion between the mineral particles of rocks 

 which have been in the zone of anamorphism; and recently this conclusion, 

 founded upon physical reasoning and observation, has been confirmed by 

 the experiments of Adams and Nicolson." They have ascertained that 

 when dry marble is deformed under pressure in all directions greater than 

 its crushing strength, no supercapillary openings form, and the deformed 

 rock has strength only a little short of that of the original marble. Since 

 the deformation was carried on while the rock was dry, the coherence of 

 the rock particles can not be attributed to the action of water, but must be 

 clue to true molecular attraction or welding. The rock was therefore 

 deformed under conditions similar to those which often obtain in the zone 

 of anamorphism. 



a Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble: 

 Philos. Trans. Royal Soc. London, ser. a, vol. 195, 1901, pp. 363-401. 



