METAMORPHISM 



345 



angles to the pressure; (2) compression also turns elongated particles 

 into parallel positions so that they take a direction in which their 

 longest axes are at right angles to the pressure; (3) as a result of 

 the metamorphism accompanying compression new minerals, such as 

 mica, are formed, and since these crystals can grow more easily 

 in the direction in which the pressure is least — along the line of 

 least resistance — they also will have their longest axes at right 

 angles to the pres- 

 sure. The combined 

 effect is to produce 

 a rock which will 

 cleave or split much 

 more readily in one 

 direction than in any 

 other. 



Since a bed of shale 

 is seldom perfectly 

 homogeneous, slate 

 differs in the perfec- 

 tion of its cleavage. 

 Sandy layers, for ex- 

 ample, are contorted 

 and poorly cleaved, 

 while the layers of 

 pure clay have a perfect slaty cleavage. Slaty cleavage will be per- 

 pendicular to the bedding if the rocks were subjected to pressure 

 when horizontal, but may be inclined at any angle to the bedding if the 

 beds were folded before the pressure became intense (Figs. 337, 338). 



The formation of slate requires much less extensive metamorphic 

 changes than does that of schist and of gneiss (p. 346). 



Schist. — Schists are rocks composed of thin, wavy leaves or 

 folia in which the foliation (Latin, foliatus, leaved) or lamination is 

 due to the abundance and parallel position of such minerals as mica, 

 hornblende, or talc. The folia are not of uniform thickness, but are 

 flattened lenses of the minerals, often bent and wavy, with their platy 

 surfaces in parallel planes. The characteristic foliated structure 

 of schists is developed when rocks have been subjected to great pres- 

 sure. Schists are the result either of (1) the formation of new min- 

 erals which developed at right angles to the pressure, since growth 

 takes place more readily along the lines of least resistance; or of (2) 



Fig. 338. — Illustration showing the relation of slaty- 

 cleavage (nearly vertical) to bedding (dipping to the 

 right). (Photo. L. E. Westgate.) 



