100 



Adventures in Scenery 



canic vents or emitted from fissures in the rocks, is molten rock. 

 Each mineral has its own melting-point. In cooling each min- 

 eral in theory should solidify as the temperature of the mass 

 lowers. This does not strictly follow in the case of molten 

 rocks, owing to the fact that the melting-point of a mineral 

 may be lower when mixed with some other mineral. For ex- 

 ample, quartz or silica alone is infusible, but when mixed with 



Photo by R. H. Finch. Courtesy Univ. of Calif. 



FIG. 28. Brokeoff Mountain, showing fault scarp. The hummocky fore- 

 ground is lava, part of the down-faulted crater of Brokeoff Volcano. 



soda or potash it is fusible. Glass is made from quartz sand 

 which is mixed with soda. The mixture of minerals in the 

 molten mass is called the magma. In a cooling magma a great 

 variety of combinations of chemical substances occurs, resulting 

 in many different kinds of igneous rocks. Thus many varieties 

 of lava occur among the volcanic or igneous rocks due to the 

 varying conditions in the cooling mass and the different min- 

 erals involved. 



