INTRUSIVE MASSES 



279 



The commonest rocks in dikes are basalt, quartz porphyry, 

 andesite, and diabase. 



When denudation has so far cut away the surface of the ground 

 as to expose the dike, the form which the latter takes will depend 

 upon the relative destructibility of the igneous rock and the enclos- 

 ing strata. If the latter wear away more rapidly, the dike will be 

 left standing above the surface like a wall (Fig. 122) ; but if the 

 igneous mass be disintegrated more rapidly than the strata, a trench 

 will mark the line of the dike. 



//C*- 





FlG. 123. — Dike of basalt cutting strata: bad lands of eastern Oregon. 



Dikes are common and conspicuous objects in the Connecticut 

 valley and in the sandstone belt which runs, with interruptions, 

 from the Hudson River to North Carolina. 



Veins are smaller and more irregular, frequently branching 

 fissures which have been filled with an igneous magma ; they may 

 be only a few inches in thickness, and may often be traced to the 

 mass which gave them off. The nature of the rock in a vein may 

 be much modified by material derived from the walls. This vein 

 rock is often so coarsely crystalline, that it has been suggested 

 that it could not have solidified from fusion, but was deposited 

 from a solution in superheated waters. 



