THE COMPOSITION OF THE EARTH 



31 



Glassy rocks. This class includes rocks composed wholly 

 or in large part of glass. 



It has already been seen that rock formed at the surface of a 

 lava flow is apt to be more or less filled with cavities formed by 

 gas bubbles. Such rocks are sometimes said to have a vesicu- 

 lar texture (vesicles = cavities). Pumice (Fig. 11) is a rock in 



FIG. 11. Pumice, about 



(Photograph by Baker.) 



which such cavities take up much of the space, and are divided 

 by very thin partitions of glassy material. Bits of pumice are 

 found distributed widely over the ocean floor, for they are often 

 floated long distances before their small pores become filled 

 with water, thus causing them to sink. As the walls of the 

 cavities become thicker and the material stony, pumice 

 grades into scoria (Fig. 12). The cavities of scoriaceous lavas 

 are sometimes partly or wholly filled at a later time by depo- 

 sition of minerals from solution in ground water (Fig. 13). 

 This, for example, is one mode of occurrence of copper in 

 some of the mines of northern Michigan. 



Obsidian or volcanic glass (Fig. 9) is a solid, glassy rock, 



