TEXTURES FORMED UNDER MASS-STATIC CONDITIONS. 699 



Under mass-static conditions the textures which are produced by 

 recrystallization are very different from those formed by crystallization 

 during mass -mechanical action. It has been seen that under mass-mechan- 

 ical conditions schists form, the distinctive textures of which are uniformity 

 of size of the mineral particles, and' especially .the similar orientation of 

 certain minerals. Under mass-static conditions, where the pressure con- 

 ditions approximate to hydrostatic, schists do not ordinarily develop. The 

 rearrangement takes place so as to form more condensed molecules, without 

 producing either uniformity of size or regularity of arrangement of the 

 mineral particles. During the development of the mineral particles they 

 ordinarily interfere with one another and interlock, producing a complex 

 texture often more intricate than that formed by the mutual interference of 

 minerals when crystallizing from a magma. 



Commonly, the old mineral particles, if large, break up into a great 

 number of other mineral particles of varying size. If, on the other hand, 

 the original particles are very small, they are apt to coalesce so as to 

 produce larger mineral particles; but usually the magnitude of the resultant 

 mineral particles is not great. To the above extent there is a tendency 

 toward uniformity of size, but it does not go so far as to result in approxi- 

 mate uniformity. Indeed, a very notable characteristic of the texture of 

 rocks recrystallized under mass-static conditions is the unequal size of the 

 mineral particles. This results from the fact that there is no movement to 

 prevent the full operation of the law of growth of large individuals at the 

 expense of small ones. In this particular the conditions contrast with those 

 of mass-mechanical action in which large individuals are especially exposed 

 to strain and rupture, and are therefore likely to be destroyed. 



This process of the growth of large individuals at the expense of small 

 ones develops a porphyritic texture. This process is of such significance 

 that it needs to be especially considered, as does also another process closely 

 allied to it, that of the regeneration or rejuvenation of old mineral particles 

 of large size which are adapted to their environment. 



DEVELOPMENT OF POEPHYRITIC TEXTURES. 



Where the mass-static conditions favorable to recrystallization continue 

 long, some of the mineral particles may grow to great size as compared 

 with their fellows, and thus a porphyritic texture is produced. The pressure 



