33 



which has a resinous surface, and is thought to resemble pitch 

 in appearance. It is variable in color, red, brown, and 

 green varieties being common. Either obsidian or pitchstone 

 may contain scattered crystals which can be recognized, giv- 

 ing rise to obsidian-porphyry and pitchstone-porphyry. 



Fragmental volcanic rocks. Volcanoes of the explosive 

 type throw out material which falls in solid fragments. 

 These are classified on the basis of size, shape, and texture. 

 Volcanic ash is composed of very small, glassy fragments. It 

 sometimes forms thick deposits about volcanoes. Still finer 

 material constitutes volcanic dust. This is scattered widely by 

 the winds, some slight amount probably having been carried 

 from certain volcanoes to all 

 parts of the world. Dust and 

 fine ashes from Iceland vol- 

 canoes settled in 1783 on cer- 

 tain farm lands in northern 

 Scotland in such quantity as 

 to destroy crops. Such ma- 

 terial from Krakatoa was 

 carried several times around 

 the earth in 1883. If the 

 material is about the size of 

 hickory nuts or medium coarse 

 gravel, it is called lapilli. 

 Cinders are made up of angu- 

 lar pieces of open texture, 

 and, together with lapilli and 

 similar fragments, form many steep-sided volcanic cones (Fig. 

 20). Masses of lava which have become more or less rounded 

 because of rapid rotation in the air are bombs (Fig. 14) . They 

 vary from the size of one's fist or less, to a diameter of several 

 feet. Volcanic breccia is a general term applied to the beds of 

 coarser material (bombs, lapilli, coarse ashes, etc.), which ac- 

 cumulate around the vent. The dust and lighter ashes settle 

 farther away to form beds of tuff. 



B. & B. GEOL. 3 



FIG. 14. Volcanic bombs, Cinder 

 Buttes, Idaho. (Russell, U.S. 

 Geol. Sure.) 



