TTNSTRATIFIED CONDITION. 



119 



and trachyte, etc., and to the columnar structure met with among 

 igneous rocks. The last is represented in the figure given above. 

 There are all shades of perfection in this columnar structure, from 

 prisms of great height with perfectly plane sides, to a mere ten- 

 dency to split in prismatic forms ; and also from this less perfect 

 prismatic character, to the massive structure with no trace of 

 columnar fracture. 



For a continuation of this subject, see the chapter on igneous 

 operations, under Dynamical Geology. 



131. (l.j General nature of veins. — The vein condition. — Veins are 

 narrow plates of rock intersecting other rocks. They are the fill- 

 ings of cracks or fissures ; and, as these cracks or fissures may either 

 extend through the earth's crust to the interior and divide it 

 for long distances, or reach down only for 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 move- 

 ments that produce joints and fractures in rocks may give origin 

 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 rocks, and have regular and well-defined walls. 



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

 calcite, fluor spar, heavy spar, etc., — ingredients which are less ob- 

 viously a liquid injection from below, and probably never of this 

 nature. They are generally irregular in form, often indistinct in 

 their walls, and very varying in their ingredients. They abound 



Fie. 116. 



Fie. 117. 



in regions of metamorphic rocks. Veins have been subdivided 

 into kinds ; but the divisions need not here be considered. 



