DISRUPTION OF IGNEOUS ROCKS 263 



or ftitclistuHt-s when dark and lustrous; perlites when a spheroidal fracture gives 

 tin-in a prurly appearam v; and pumice when greatly inflated by included . 



IV. I'yroclaslic rocks are 



TujL if (.imposed of finely comminuted pyroclastic material; 



Volcanic breccia if composed of coarse angular pyroclastic materials. Agglom- 

 cri/.'c is a term much used for volcanic breccia, and for similar rock whose con- 

 stituents are but little rounded. If the constituents are well rounded, the rock 

 In-romes volcanic conglomerate. 



In general discussions, it is serviceable to use the term granitoids in a broad 

 generic sense, to include all crystalline rocks of the general granitoid type, including 

 the granites, syenites, etc. In a similar broad way, gabbroids may be used to 

 include the dark crystalline rocks in which the ferromagnesian minerals predomi- 

 nate, as the diorites, gabbros, dolerites, peridotites, etc. In this convenient and 

 comprehensive way, two contrasted groups of igneous rocks may be designated. 

 As the granitoids are usually acidic and the gabbroids basic, the grouping repre- 

 sents a broad fact of importance. 



> 

 The Disruption of Igneous Rocks 



At the surface, igneous rocks are subject to mechanical dis- 

 ruption, and to chemical change which results in decay. 



Mechanical disruption. One great agent of mechanical disrup- 

 tion at the surface is change of temperature. This has been dis- 

 cussed in Chapter II and other phases of mechanical disruption 

 arc discussed in Chapters IV and V. All mechanical disruption of 

 igneous rock leaves the fragments essentially like the original rock 

 in composition. 



Chemical disintegration. Most of the silicate minerals which 

 make up the larger part of all igneous rock are complex, chemi- 

 cally. Not a few of them contain as many as three or four basic 

 elements, in union with oxygen and silicon. Substances which are 

 complex chemically, are, as a rule, less stable than those of simple 

 constitution. Complex silicates, such as the feldspars, micas, 

 amphiboles, and pyroxenes tend to break up into simpler sub- 

 stances. Chemical changes are helped along by the oxygen, carbon 

 dioxide (CO 2 ), and water vapor of the air, and by water after it is 

 precipitated. Some of the simpler changes may be noted. 



Oxygen may enter into combination with the iron of a silicate 

 mineral containing iron. The iron is thus taken out of its silicate 

 combination, and in union with the oxygen forms iron oxide, a 

 simple and stable chemical compound. This process is oxidation. 

 Oxidation affects other elements also. 



Similarly, carbonic dioxide from the air may enter into com- 

 bination with the base of a silicate mineral. Thus it enters into 



