688 A TREATISE ON METAMORPHISM. 



numerous porphyritic, simple twinned albites, with a close approximation 

 to definite orientation, the two greater dimensions of the crystals lying- in 

 the planes of schistosity. Similar phenomena in reference to feldspar are 

 shown by some of the augen-gneisses. Not infrequently porphyritic mica 

 and chloritoid show approximately similar orientations. In some cases 

 porphyritic staurolites have their greater dimensions arranged approxi- 

 mately in the schistose planes. In such cases as these the fortunate 

 positions of the porphyritic crystals are such that the tendency for large 

 individuals to grow at the expense of small ones is sufficiently strong to 

 prevent the usually dominant tendency- to destroy large individuals, and 

 thus to prevent the production of the even-grained texture which is normally 

 characteristic of the schists produced during mass-mechanical movement. 



In the case of an isometric mineral, such as garnet, which has no 

 cleavage and an isometric habit, it may be possible for porphyritic crystals 

 to develop during mass-mechanical movement, although it is believed not 

 to be common at least for undistorted crystals, for such growth would imply 

 that minerals of this kind are able to grow as far against the greatest presr 

 sure as in the direction of least pressure. 



The second characteristic feature of the recrystallized slat}^, schistose, 

 and gneissose rocks is that the mineral particles show a marked tendency 

 toward regular orientation. This orientation may consist in the particles 

 having major, mean, and minor diameters in approximately common direc- 

 tions, or in certain species having their crystallographic axes in nearly 

 common directions, as a result of which the like cleavages of all the 

 particles of a given mineral are approximately in the same plane, or in 

 the two combined. (PI. XI, C.) Orientation, where marked, gives the 

 rocks a cleavage. 



The most important of the minerals the particles of which show 

 similar crystallographic orientation are the micas, especially biotite and 

 muscovite. With these minerals similarity of orientation is usual. Another 

 set of minerals the particles of each of which frequently show a marked 

 tendency toward similar crystallographie orientation are chlorite, amphi- 

 bole, and feldspar. Other less important minerals are known to show the 

 same phenomena. Of course, it is understood that the crystallographic 

 orientation is in no case perfect, but with the micas it may approach per- 

 fection. From the extreme of regularity of orientation shown by mica in 



