MEANING OF ROCK FLO WAGE. 757 



and their mean axes are in the direction of mean pressure. When once 

 the mineral particles have oriented themselves in this way, they continue 

 to grow according to their crystal habits, the most rapid growths being in 

 the direction of greatest pressure. This dimensional arrangement control- 

 ling the orientation of mineral particles forming in rocks during conditions 

 of unequal stresses has been formulated by Leith as a result of his. detailed 

 studies of rock cleavage." The amount of mass shortening which results in 

 the regular arrangement may be comparatively slight, and yet the differ- 

 ences in the dimensions of the mineral particles be great. For instance, 

 Leith's measurements show that the common ratio between the least and 

 greatest diameters of the mica flakes of mica-schists is, on the average, 

 about 1 : 10. 6 It does not follow, as has been thought by some, that this 

 involves the shortening of the rock mass by 0.9. Indeed, it is perfectly 

 conceivable that the rock mass may be but slightly shortened, perhaps by 

 0.1, and yet produce mineral particles the shortest dimensions of which are 

 only 0.1 as great as their longest dimensions. During recrystallization of 

 a rock the material of those minerals which have strong crystal habit 

 rapidly attaches itself to the borders of the particles and but slowly to the 

 sides. 



The above conclusion seems to be completely confirmed by the con- 

 stancy of the ratios between the greatest and least diameters for a given 

 mineral. Leith has ascertained that in mica the ratio varies from 1 00 : 65 

 in the case of biotite to 100 : 10 in muscovite, with an average of 100 : 20 

 for biotite and 100:14 for muscovite; in hornblende from 100:40 to 

 100 : 25; in feldspar from 100 : 75 to 100 : 50.° In the case of each mineral 

 these ratios are practically the same whether the rock be a slate, a fine- 

 grained schist, or a coarse-grained gneiss. The full range of variation in 

 the ratios of a mineral can be found in the same rock in each of the dif- 

 ferent stages of metamorphism. If the ratios between the least and greatest 

 diameters of the mineral particles were taken as evidence of the amount of 

 shortening of a rock, we should reach a different conclusion for each 

 mineral and the same conclusion for all stages of metamorphism. Such a 

 conclusion is absurd, and the independence of the dimensional ratios of the 

 orienting minerals and the amount of rock shortening is perfectly estab- 

 lished. Therefore, so far as new mineral particles are produced with similar 



"Leith, C. K., Bock cleavage. b Leith, C. K., cit. c Leith, C. K., cit. 



