24 INTRODUCTION 



represents on a minute scale what is occurring in the lakes and oceans 

 of the earth. In the pool the deposit may be only a fraction of an 

 inch thick, while off the shores of the ocean it may be many thou- 

 sands of feet in depth. When, as has often occurred in the past, 

 such an enormously thick deposit has been raised above the sea and 

 streams have cut deep valleys into it, it is seen to be made up of layers, 

 and the rocks composing it are consequently called stratified (p. 233) 

 or layered rocks (Fig. 1). The planes which separate the layers from 

 one another are called bedding planes or planes of stratification. If the 

 rock is made of hardened mud it is called shale, and is usually divided 

 into many thin layers or lamina?, the laminae being separated by 

 planes of bedding. If the rock is composed of sand whose grains are 

 cemented together by lime or other substances, it is called a sandstone. 

 Sandstone is also stratified, but the bedding planes are usually farther 

 apart than in shale. Sedimentary rocks are not always in the 

 horizontal position in which they were deposited, but are often folded 

 and tilted. 



Igneous Rocks. — Although few persons have seen lava flowing 

 from a volcano, many have seen the molten slag or glass of blast 

 furnaces, which bears a resemblance to lava after hardening, and is, in 

 fact, not unlike the lava of some volcanoes, both in composition and 

 structure. Lava (p. 298) is an igneous rock (Latin, ignis, fire) ; 

 that is, a rock which has been in a molten condition. The majority 

 of igneous rocks, however, are not glassy, but are composed of dis- 

 tinct grains or crystals. This crystalline structure, as we shall learn 

 (p. 302), is brought about when molten rock cools so slowly that 

 time is given for crystals to form. An igneous rock is, therefore, one 

 which solidified from a state of fusion ; it is either glassy or grained 

 (crystalline). 



It is apparent, therefore, that igneous rocks differ from sedimentary 

 in a number of particulars ; the former are either glassy or crystalline 

 and are devoid of stratification planes, while sedimentary rocks are 

 seldom crystalline and are arranged in layers. 



Granite is a typical crystalline, igneous rock (Fig. 5, p. 29) and is 

 composed, usually, of three minerals, the most conspicuous of 

 which is feldspar. These feldspar grains or crystals are opaque, and 

 white, pink, or gray in color. Mica, when present in granite, is usually 

 easily recognizable by its glistening leaves which split into elastic 

 scales. The third conspicuous mineral of granite is quartz. It is 

 usually colorless, has the appearance of broken glass, and is harder 



