The Lava Plain of the North 101 



All, or nearly all, the chemical elements known on the earth 

 are found in igneous rocks. Of the 70 -odd chemical elements 

 known only eight are of such importance that they need to be 

 considered in a general study of the igneous rocks. From these 

 elements various chemical combinations arise resulting in the 

 formation of the various minerals, and out of the mixtures of 

 minerals come the various rocks. In importance oxygen is at 

 the head of the list of elements. Next to oxygen, silica, SiCX, 

 is the most important of the rock-forming agencies. Silica is 

 the oxide of silicon. (Silicon is very rare in nature.) The 

 union of oxygen with the six more common elements forms 

 oxides, and the oxides are fundamental in the rock-forming 

 processes. Of these oxides silica acts as an acid while practi- 

 cally all the rest act as bases. An acid and a base combine 

 chemically to form a salt. The acid silica combines with the 

 basic oxides to form salts, which are called silicates. Thus in 

 general igneous rocks are silicates of the six leading basic oxides, 

 viz., alumina, potash, soda, lime, magnesia and iron. This is 

 the general fact, but in nature there are many complications 

 in the combinations of minerals, and the total number of sili- 

 cate minerals is very large. These become the problem of the 

 mineralogist. The student need not feel embarrassment or dis- 

 couragement if he is unable to recognize the constituent min- 

 erals of the crystalline rocks. Their classification is technical 

 and somewhat difficult, and is the task of the professional 

 mineralogist. 



It is not known that any volcanoes existed on the continent 

 of North America before the Cretaceous period of geologic 

 time. Molten rocks that have been forced out of fissures in 

 the crust of the earth are the same in kinds as those of volcanic 

 origin, and are classed as igneous. Rocks that have emanated 

 from the depths of earth's interior are often spoken of as plu- 

 tonic. Those that have been poured out on the surface are 

 lavas. Volcanoes reached their maximum at the close of Cre- 

 taceous and during Tertiary time. There were tremendous 



