32 EARTH FEATURES AND THEIR MEANING 



mine, particularly with the aid of the polarizing microscope, 

 whether the original rock from which it was derived was of igneous 

 or of sedimentary origin. There are, however, many examples 

 which have defied a reliable verdict concerning their origin. 



Characteristic textures of the igneous rocks. In addition to 

 the massiveness of their general aspect and the angular bound- 

 aries of their constituents, there are many additional textures 

 which are characteristic of the igneous rocks. While those that 

 have consolidated below the earth's surface, the intrusive rocks, 

 are notably compact, the magmas which arrive at the surface of 

 the lithosphere before their consolidation reveal special structures 

 dependent either upon the expansion of steam and other gases 

 within them, or upon the conditions of flow over the earth's sur- 

 face. Magmas which thus reach the surface of the earth are de- 

 scribed as lavas, and the rocks produced by their consolidation 

 are extrusive or volcanic rocks. The steam included in the lava 

 expands into bubbles or vesicles which may be large or small, 

 few or many. According to the number and the size of these 

 cavities, the rock is said to have a vesicular, scoriaceous, or pumi- 

 ceous texture. 



Most lavas, when they arrive at the earth's surface, contain 

 crystals which are more or less disseminated throughout the 

 molten mass. The tourist who visits Mount Vesuvius at the time 

 of a light eruption may thrust his staff into the stream of lava 

 and extract a portion of the viscous substance in which are seen 

 beautiful white crystals of the mineral leucite, each bounded by 

 twenty-four crystal faces. It is clear that these crystals must 

 have developed by a slow growth within the magma while it was 

 still below the surface, and when the inclosing lava has con- 

 solidated, these earlier crystals lie scattered within a groundmass 

 of glassy or minutely crystalline material. This scattering of 

 crystals belonging to an earlier generation within a groundmass 

 due to later consolidation is thus an indication of interruption in 

 the process of crystallization, and the texture which results is 

 described as porphyritic (Fig. 17 6). Should the lava arrive at 

 the surface before any crystals have been generated and consoli- 

 date rapidly as a rock glass, its texture is described as glassy 

 (Fig. 17 c). 



When the crystals of the earlier generation are numerous and 



