328 



DYNAMICAL GEOLOGY. 



C4) By the disruptive or expansive action of vapors : as of vapors attending volcanic- 

 action ; resulting not only in fissures, but also in vesicles or cavities in an ejected igneous 

 rock, or along the walls of the dikes. 



(5) Corroding vapors or solutions rising in a fissure have sometimes enlarged the 

 fissure in some parts or made open spaces, especially when the rock was a limestone ; 

 thus large chambers have been excavated in this yielding rock for the reception of min- 

 erals and ores. 



(6) Porous strata have taken in vein-material in proportion to their porosity. 



(7) Cave7'ns, however made, have become occupied with vein-material. 



2. Forms and Kinds of Fissures. 



Fissures intersect strata vertically or obliquely or make a network. The 



angle whicli the plane of a vein makes Avith the vertical is called the hade of 



the vein ; the hanging wall is the upper wall in an oblique vein, and the 



opposite is the foot wall. 



287. 



285. 



286. 



Veins. — Fig. 2S5, two simple veins; 286, two veins, one faulted; 287, a network of quartz veins intereecting" 



schist, the slab 5 feet square. 



In the case of upturned rocks, veins may either cut across the beds, or 

 occupy spaces between them. Such interstitial veins (Figs. 287, 288, 290) 

 are very common in slaty and schistose rocks, because forces below can more 

 easily open such spaces than make fractures across the beds ; for it is fol- 



288. 



289. 



290. 



291. 



Interstitial veins. 



lowing the grain of the rock. Such an opened space may continue for some 

 distance between the bedding, and then cut across to another plane of bed- 

 ding, and so on, the mean direction being that which the space would have 



