332 



PHYSICAL GEOLOGY 



Fig. 327. — A Hand specimen of obsid- 

 ian, showing the characteristic conchoi- 

 dal fracture. (U. S. National Museum.) 



Obsidian or Volcanic Glass (Fig. 

 327) is pure, natural glass, entirely 

 or nearly devoid of crystal grains. 

 It is usually jet black in color, but 

 is sometimes gray, green, red, or 

 yellow. Because of the sharp edges 

 which form when it is broken, it was 

 highly prized by the Mexicans and 

 other primitive peoples for the 

 manufacture of sharp implements, 

 such as knives and arrowheads. 



Pitchstone is a variety of ob- 

 sidian in which the luster is resin- 

 ous or pitch-like. The chemical 

 and other differences between this 

 rock and obsidian are slight. 

 Pitchstones are variable in color. 

 When conspicuous crystals are 

 scattered through the rock, it is 

 called pitchstone -porphyry. 



Fragmental Volcanic Rocks 



Rocks formed from the material thrown out by volcanoes are in- 

 cluded under this head and are made by the consolidation of dust, 

 ashes (material the size of a shot), lapilli (the size of a nut), and bombs 

 (pieces the size of an apple, or larger). 



Tuff. — When the rock is composed entirely of the finer kinds of 

 volcanic detritus, it is called volcanic tuff. Rocks of this type are 

 light in weight and usually loose in texture, although some are almost 

 as compact as felsites. Tuffs contain fossils if the dust and ashes 

 of which they are composed fell on a land surface covered with vege- 

 tation, or in water in which marine organisms were living. Some of 

 the rock through which the Panama Canal was cut is a tuff containing * 

 marine shells. Tuffs are widely used in Mexico for building stones. 



Volcanic Breccia. — This is a rock composed of angular fragments 

 of volcanic rock, bombs, etc., which are cemented together with ash 

 and dust. 



a more detailed study of igneous rocks the student is referred to L. V. Pirsson, Rocks 

 ,111(1 Rock Minerals; and J. r. Kemp, Handbook 0/ Rocks. 



