64 OUR PHYSICAL WORLD 



Still there are regions in which the very old rocks are found at 

 the surface, later sedimentary rocks having been washed off 

 from them; or else they have been brought to the surface by 

 the folding and crumpling of the earth's crust. In mountain 

 regions where volcanic activity is present, igneous rocks are very 

 plentiful. 



When a great mass of molten material like a great lava out- 

 flow cools, the surface layers cool first, naturally. These surface 

 layers are made up of the lighter materials which have come to 

 the top while the mass was still molten. Moreover, such molten 

 masses are full of gases that are escaping and bubbling up to the 

 surface. The rock, therefore, that first forms on the top of 

 such a lava mass is likely to be frothy, light in color, and light 

 in weight. Deeper down in the cooling mass the rock formed is 

 glassy in its texture. Still deeper down as cooling goes on much 

 more slowly, the ingredients of the molten mass crystallize as 

 they cool. When cooling goes on fairly rapidly, crystals that 

 form are very small; but as cooling goes on more and more 

 slowly the crystals tend to become larger and larger. It is very 

 evident that igneous rocks will vary in their structure according 

 to the rate at which the original molten mass cooled. We may 

 have rocks of spongy character, like pumice, glassy rocks such 

 as obsidian, or crystalline rocks, and these latter may be either 

 fine-grained like basalt or coarse-grained like gabbro. The 

 coarsely crystalline rocks of all groups are called plu tonic, for they 

 have been formed, as a rule, deep down in the throat of the 

 volcano. The finely crystalline, glassy, porous rocks are called 

 volcanic, for they have cooled upon the surface of the earth 

 as lava outflows. 



The various minerals that enter into the composition of the 

 rocks do not crystallize out all at the same time. Some begin to 

 form as crystals when the molten mass is still quite hot. Others 

 wait until the material has cooled a great deal. The minerals 

 that contain large proportions of the heavier metallic elements, 

 such as iron and magnesium, crystallize early. Plagioclase 



