218 The Realm of Nature CHAP. 



friction produces heat enough to soften the substance which 

 is rolled out, so that the original structure disappears, the 

 minerals are altered chemically, and the rock acquires a 

 flaky texture and is known as a schist. The change may 

 produce a crystalline structure very similar to that of granite 

 as in the rock called gneiss. Local or contact metamorphism 

 is brought about by an intrusive sheet of liquid igneous rock 

 forcing its way between other strata and altering their com- 

 position and physical state. The edges of sandstone may 

 thus be fused into glassy quartzite, and soft clay beds baked 

 into a hard porcelain-like mass. 



290. Dip, Cleavage, Joints, and Faults. Sedimentary 

 rocks are sometimes raised by upheaval so steadily and 

 uniformly that the strata remain horizontal, but far more 

 commonly the strata are inclined in a particular direction. 

 The inclination of a bed of rock to the horizon is called 

 its dip, and is measured by the angle FAB (Fig. 42). 

 Rocks are found dipping at all angles, sometimes as high 



FIG. 42. Illustration of rock structures. AB, horizontal line ; FF, fault ; S, 

 slaty cleavage ; J, joints. The long parallel lines mark planes of bedding 

 making an angle of 19 with the horizontal. 



as 90 ; then the strata stand upright. The stresses which 

 elevate rocks usually act horizontally as a thrust from two 

 sides, and the particles of the rock sometimes yield and are 

 flattened out. When this happens the rocks split up more 

 readily along the flattened sides of the particles than along 

 its original planes of bedding, and are said to have acquired 

 cleavage-planes (oblique lines S in Fig. 42). The cleavage- 



