GEOLOGY 



2435 



GEOLOGY 



the distance below the surface at which it may 

 be found, is the water table. In a dry season 

 the water table is lowered, while in a wet sea- 

 son it is raised. Ground water holds various 

 sorts of plant food and other substances in 

 solution. It is the chief source of nourish- 

 ment for plants, and it also aids in the decom- 

 position of rocks. It is, therefore, one of the 

 agents whose work we need to remember when 

 considering the changes now taking place on 

 the earth. 



The Lithosphere. The lithosphere, or rock 

 sphere, includes the solid portion of the earth; 

 this is about eight-ninths its moss, by which 

 term, however, we do not mean its thickness. 

 Geologists usually speak of it as the earth's 

 crust. As the story progresses we shall find 

 that geology is closely related to other sciences, 

 especially astronomy, botany, zoology, chem- 

 istry and physics. Its subject-matter overlaps 

 that of physiography at many points, and here 

 the article PHYSIOGRAPHY should be referred to. 



Reading the Rocks 



Specimens. Tom's question about the shell 

 and the teacher's method of answering it 

 awakened a lively interest in rocks, and the 

 teacher's desk was soon covered with speci- 

 mens brought in by the pupils. Nearly all 

 these specimens were gathered in the locality, 

 yet some showed marked differences in color 

 and structure. Some contained shells like 

 Tom's; others contained shells of a different 

 kind; in others were found objects that looked 

 like twigs turned to stone. Still others re- 

 vealed beautiful crystals and some had a 

 glassy appearance. A similar collection can 

 be obtained in almost any locality. 



Classification. After the boys and girls had 

 examined the specimens for themselves the 

 teacher began his lessons. He asked the pupils 

 to arrange them first into two groups, the hard 

 and the soft rocks, and then to subdivide the 

 hard rocks into those built up of rounded 

 water- or wind-worn grains, those composed 

 of crystalline grains, which show no wear, and 

 a third class, those having a glassy appear- 

 ance. The rounded grains of the first class, 

 he pointed out, may be compared with the 

 rounded grains of sand in rivers and seas and 

 on deserts. From this comparison the pupils 

 were led to conclude that originally these hard 

 rocks were loose sands which later became 

 consolidated. They are therefore called sand- 

 stones. The teacher went on to say that when- 

 ever they are so thoroughly consolidated as to 

 split through the grains rather than around 

 them, they are called quartzites. 



All the glassy rocks, the pupils learned, were 

 formed from a molten state only, and the hard 

 non-fragmental "granular rocks were once 

 molten also. The grains of these interlock, 

 and some have a crystal outline. These char- 

 acteristics of grain develop only when rocks 

 solidify from a molten or dissolved condition. 

 Because great heat is necessary to make rocks 



molten they are called igneous rocks. Igneous 

 comes from a Latin word meaning fire. Such 

 rocks may be briefly defined as hardened lavas. 

 See IGNEOUS ROCKS. 



The pupils were then directed to test the 

 soft rocks with muriatic acid. Those which 

 gave off a gas they learned were limestones, 

 the gas given off being carbon dioxide. The 

 soft rocks which did not give off gas under 

 the application of muriatic acid broke along 

 parallel surfaces. The pupils noticed that these 

 were fine grained, and that some were black 

 and others gray and brown. Such rocks are 

 called shales or slates, the slates being more 

 compact than shales. The teacher told the 

 pupils that shales and slates result from the 

 consolidation of muds. 



Next the pupils""were told to look for fos- 

 sils. These, the teacher explained, are relics 

 of the plants and animals of a former era, 

 which have been buried by natural causes in 

 certain kinds of rock. They are to be found 

 in the sandstones, quartzites, limestones and 

 shales or slates, but not in igneous rocks. By 

 a careful study of the specimens containing 

 the fossils, and with the help of the teacher, 

 the pupils arrived at the conclusion that these 

 rocks must have formed from sediment that 

 settled on the bottom of some body of water, 

 slowly burying the plants and animals whose 

 fossils the pupils had just discovered, and at 

 the time these objects were buried the rock 

 must have been soft mud. In time this mud 

 dried and hardened into rock. Since these 

 rocks were formed from sediment they are 

 called sedimentary rocks, and since such rocks 

 are usually formed in layers they are also 

 called stratified rocks. 



Each of the specimens was examined with 

 a magnifying glass and its color and peculiari- 

 ties were noted. In this way the principal 

 minerals which form the rocks were learned. 



