NATURE OF THE MATERIALS IN THE LITHOSPHERE 27 



in the vast number of instances it is the behavior, and not the 

 appearance, of the mineral substance which is made use of for iden- 

 tification. 



When a mineral is broken under the blow of a hammer, in- 

 stead of yielding an irregular fracture, like that of glass, it generally 

 tends to part along one or more directions so as to leave plane 

 surfaces. This property of cleavage is strikingly illustrated for 

 a single direction in the mineral mica, for two directions in feld- 

 spar, and for three directions in calcite or Iceland spar. Other 

 properties of minerals, such as hardness, specific gravity, luster, 

 color, fusibility, etc., are all made use of in rough determinations 

 of the minerals. Far more delicate methods depend upon the 

 behavior of minerals when observed in polarized light, and such 

 behavior is the basis of those branches of geological science known 

 as optical mineralogy and as microscopical petrography. An out- 

 line description of some of the common minerals and the means 

 for identifying them will be found in appendix A. 



The alterations of minerals. By far the larger number of 

 minerals have been formed in the cooling and consequent con- 

 solidation of molten rock material such as during a volcanic erup- 

 tion reaches the earth's surface as lava. Beginning their growth 

 at many points within the viscous mass, the individual crystals 

 eventually may grow together and so prevent a development of 

 their crystal faces. 



Another class of minerals are deposited from solution in water 

 within the cavities and fissures of the rocks ; and if this process 

 ceases before the cavities have been completely closed, the minerals 

 are found projecting from the walls in a beautiful lining of crys- 

 tal the Krystallkeller or " crystal cellar." It is from such 

 pockets or veins within the rocks that the valuable ores are ob- 

 tained, as are the crystals which are displayed in our mineral 

 cabinets. 



There is, however, a third process by which minerals are formed, 

 and minerals of this class are produced within the solid rock as 

 a product of the alteration of preexisting minerals. Under the 

 enormous pressures of the rocks deep below the earth's surface, 

 they are as permeable to the percolating waters as is a sponge 

 at the surface. Under these conditions certain minerals are 

 dissolved and their material redeposited after traveling in the 



