INTRODUCTION 



25 



than steel. A crystalline igneous rock is, therefore, made up of a 

 number of minerals differing in color, in hardness, and in chemical 

 composition. The importance of this character will be seen when the 

 effect of the weather upon rocks is studied. 



Metamorphic Rocks are those which have been more or less pro- 

 foundly changed from their original condition by heat and pressure, 

 and are usually crystalline in texture. Most metamorphic rocks 

 possess a cleavage which 

 causes them to break easily 

 in one direction. They are 

 derived both from igneous 

 and from sedimentary rocks. 

 Metamorphic rocks have 

 parting planes like sedimen- 

 tary rocks,' and a crystalline 

 structure like igneous rocks. 



Divisional Planes. — All 

 rocks are more or less broken 

 by planes which separate 

 them into blocks. An ex- 

 amination of a sandstone or 

 limestone quarry will show 

 that, in addition to the bed- 

 ding planes, the rock is 

 broken by two or more sets 

 of fissures which run at right 

 angles to the bedding. These 

 are called joints (Fig. 1). 

 Joints occur also in igneous 

 rocks, some often being ap- 

 proximately horizontal and others tending towards the vertical. 



When beds are displaced along a joint or other crack so that the 

 strata on the opposite sides of it do not match, the beds are said to be 

 faulted (p. 261) (Fig. 2). 



Fig. 2. — A fault. The thin-bedded band 

 was once continuous. (U. S. Geol. Surv.) 



