CHARACTERISTICS OF ROCKS. 65 



Again, by the examination of thin slices of crystalline rock with the 

 microscope, it may be ascertained whether the mass of the crystalline 

 rock consists throughout of distinct grains crystalline in nature, or 

 whether portions among the stony grains are of the nature of glass. 

 Glass is a result of comparatively rapid cooling, too rapid for the 

 crystallization of the whole of the mass ; and in the glassy portions 

 the material of the rock is said to be unindividualized, the compo- 

 nent minerals of glass not being distinguishable. 



Again, igneous rocks sometimes exhibit under the microscope a 

 fluidal texture; that is, the material, when examined in sections, shows 

 wavy lines or bands, which are evidence of a former fluid state, and of 

 movement or flowing when in that state. Again, the crystals in the 

 mass are sometimes found to be broken, as an effect of motion in the 

 fluid mass after it was partly consolidated. 



5. Hie Specific Gravity of the Rock. — This quality will vary in the 

 same rock with the proportions of the constituents. If a rock consist 

 chiefly of quartz, or of feldspar, the specific gravity will be between 

 2*4 and 2*75 ; but if it consist of either of these minerals along w r ith 

 the iron-bearing mineral hornblende, or augite, or epidote, the specific 

 gravity may vary from 2-75 to 3*4, according to the proportions. 



6. Hardness, Firmness, Toughness, Durability. — These are among the 

 qualities referred to in describing rocks. But to a large extent the 

 characters belong to some varieties of certain kinds of rocks, and not 

 to any rocks in all their varieties. Quartz makes the hardest of rocks, 

 and talc and gypsum the softest. At the same time many rocks con- 

 sisting of quartz have little firmness or none. Firmness depends 

 largely on degree of consolidation and closeness of texture. Some 

 quartz rocks are very friable (rubbing to pieces easily in the fingers), 

 and some beds of the same material are too soft to be called even 

 sandstone. Granular limestone may vary in like manner, the rock of 

 some beds being firm, and of others not far distant, fragile, if not 

 friable. 



Tough rocks are not made of minerals that have a vitreous or 

 glassy lustre, with great hardness, for in such there is too much elas- 

 ticity for toughness ; but of those that yield somewhat under the ham- 

 mer and are sub-vitreous or almost earthy in lustre. Hornblende 

 makes the toughest of rocks, and to this the word horn in the name 

 alludes ; and it is especially effective when its crystallizations in the 

 rock are oblong and interlace with the other grains present. Next to 

 hornblende is serpentine, a mineral not harder than limestone. 



Durability is due largely, other things being equal, to a fineness 

 of grain and close compactness of texture sufficient to resist the effects 

 over the surface of changes of temperature, and exclude moisture, 



