MATERIALS OF THE EARTH 21 



ties of volcanic dust, etc., (sometimes called volcanic ash) become 

 coherent, as by cementation, the resulting rock is called tuff (or 

 volcanic tufa). If the constituents are largely coarse, instead of 

 fine, the resulting rock is volcanic agglomerate. Pyroclastic rocks 

 are less abundant than other volcanic rocks. 



The nature of lava. We commonly think of lava as melted 

 rock, but this view is hardly the correct one. Lava is rather a 

 solution of mineral matter in mineral matter. A simple analogy 

 may illustrate what is meant. If pounded ice and salt are mixed 

 together at a temperature of 30 F., the two form a liquid, though 

 the temperature is too low to melt either one. We commonly say 

 the salt is dissolved, but it would be just as correct to say that the 

 ice is dissolved. The ice did not melt, because the temperature 

 was too low to melt it. The two minerals, ice and salt, are dissolved 

 in each other, and the solution takes place at a temperature below 

 the melting point of either. Something of the same sort appears 

 to take place when rock becomes liquid. The liquid form is prob- 

 ably assumed at a temperature below that which would be necessary 

 to melt the minerals which make up the lava. 



The propriety of regarding lava as a solution of minerals in 

 minerals is shown in another way. Granite, an igneous rock, is 

 made up chiefly of three minerals, quartz, feldspar, and mica. These 

 minerals melt at .different temperatures. If lava were nothing more 

 than mineral matter melted, the several minerals should solidify 

 from the liquid as it cools in the reverse order of their fusibility. Of 

 the above minerals, quartz would be the last to melt, and if lava 

 were merely melted rock, quartz should be the first to take the 

 solid form as the lava cools, and it should become solid when the 

 lava reaches the fusing temperature of this mineral. But this is 

 not what happens. The least fusible mineral may be the last to 

 take the solid form. In other words, the order in which the various 

 minerals solidify from the liquid lava is independent of their fusibi- 

 lity. This would not be the case if they were simply melted. 



The liquid lava is essentially a fluid glass. It is analogous to 

 common glass, which is a silicate of potash, soda, or other base, 

 except that manufactured glass is relatively free from iron and 

 other coloring substances which abound in the lavas, rendering them 



