UNSTEATIFIED CONDITION. 



109 



reach down only to a limited depth, or be confined to single strata, 

 so veins are exceedingly various in extent. They may be no thicker 

 than paper, or they may be scores of rods in width, like the great 

 fissures opened at times to the earth's inner regions by subterranean 

 agency. They may be clustered so as to make a perfect net-work 

 through a rock, or may be few and distant. And, as strata have been 

 faulted, so veins also may have their faults or displacements. All 

 those subterranean movements that produce joints and fractures in 

 rocks may give origin and peculiarities to veins. 



(2.) Subdivisions. — Veins are divided into dikes and proper veins. 



Dikes are filled by volcanic rocks, basalt, trap, or some other ig- 

 neous rock, and have regular and well-defined walls. 



Veins are occupied by quartz, granitic rocks, metallic ores, calcite, 

 fluor spar, barite, etc., — material which is less obviously a liquid 

 injection from below, and probably is seldom of this nature. They 

 are generally irregular in form, often indistinct in their walls, and 

 very varying in their ingredients. They abound in regions of meta- 



Fig. 116. 



Fi ff . 117. 



morphic rocks. Veins have been subdivided into kinds 

 divisions need not here be considered. 



but the 



(3.) Forms and faults of veins and dikes. — Fig. 116 represents two 

 simple veins or dikes {a a and b b) intersecting stratified rocks. 

 Fig. 117, a net-work of small veins. 



Fig. 119. 

 Fig. 118. 



JDCD 



Fig. 118, small veins of quartz intersecting gneiss, — the mass five feet square. 

 The veins do not all cross one another, and correspond to the cracks which result from 

 contraction, as by sun-drying or cooling, rather than to those of any other mode of 

 fissuring. 



