MEANING OF ROCK FLO WAGE. 753 



study of the slates, schists, and gneisses I am inclined to believe that 

 shortening in one direction and unequal elongation in the direction at right 

 angles to this is the most common case, though my thin sections give 

 illustrations of all the cases. The particles are arranged with their greater, 

 mean, and minor diameters corresponding to least, mean, and greatest 

 pressures. In proportion as the movement involves shearing motion or 

 scission," the mineral particles are rotated from a position in which the 

 direction of greatest elongation is at right angles to the direction of greatest 

 pressure, although at any given time the mineral particles tend to develop 

 with their longer axes at right angles to the maximum pressure. 



In some cases the direction of greatest pressure varies within exceed- 

 ingly short distances. This is illustrated by large rigid particles, such as 

 feldspar, garnet, and other refractory minerals, which act as transmitters 

 of pressure to more than an average extent. Adjacent to the rigid 

 mineral particles the direction of greatest pressure is modified from point 

 to point, thus deviating from the averag'e for the rock. New minerals 

 forming may curve about the rigid granules. This is beautifully illus- 

 trated by the flat, curved quartzes adjacent to feldspar in the quartz- 

 porphyry described by Futterer. (PI. Ill, B; see p. 738.) In this case 

 the flat individuals lack orientation and show undulatory extinction inde- 

 pendently of subsequent strain. 



' The above process of flattening by recrystallization is general ; and 

 therefore we conclude that while recrystallization is constantly occurring 

 in the deformation of rocks, at any given time a large number of the 

 mineral particles retain their integrity, and are nuclei which at any moment 

 orient the material being deposited. In many rocks evidence may be 

 seen that this has happened. The old mineral particles, represented by the 

 cores, are partly altered, and in consequence of this may be discriminated 

 from the freshty added material; or the cores show a border of iron oxide 

 or other mineral; or the old and new materials have slightly different 

 compositions, and this may be discovered by a difference in the color, 

 refraction, or extinction, or in some other way. Finally, all of the old 

 mineral particles may be regenerated or recrystallized. 



« Becker, G. F., Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soc. America, 

 vol. 4, 1893, pp. 24-25. 



HON XLVII — 04 48 



