MATERIALS OF THE EARTH 23 



during such time of quiet, a renewal of eruption may take place, 

 and the whole mass of lava may be shifted into new surroundings, 

 and a sec.ond phase of solidification may be superposed on the one 

 already started. The rock may then show two phases of crystal- 

 lization: (1) Large crystals of the kind or kinds which developed 

 in the lava during the first stage of slow subterranean cooling; and 

 (2) small crystals or glass developed during the more rapid cooling 

 under the new conditions. The result is large crystals set in a 

 matrix of small crystals or of glass. Rock of this physical compo- 

 sition is porphyry, a term which has a textural but not a mineral- 

 ogical significance. 



Composition of Igneous Rocks 



Nearly all the chemical elements known on the earth are found 

 in igneous rocks, though but few of them are abundant. These 

 few are regarded as the normal or essential constituents, while the 

 rarer substances are regarded as incidental. The relative amounts 

 of the more abundant elements in the crust of the earth, as nearly 

 as now known, are shown in the following table: 1 



It will be seen that only eight of the elements exceed one per 

 cent, and no other one reaches one-half of one per cent. Many 

 of the elements that are of the utmost importance in the affairs of 

 men occur in quantities too small to be estimated in percentages. 

 The precious metals, and even some of the more common ones, 



1 Clarke, F. W. Bull. 168, U. S. Geol. Surv. 



