THE ORIGIN AND DESCENT OF ROCKS 



437 



depositing. Very large cavities lined in this way are known to miners 

 as vuggs, and these grade on into caves lined with crystals and with 

 stalactite and stalagmite. These are the largest expression of the solidify- 

 ing process by means of internal deposition. 



Fissure-filling ; veins. — Cracks, crevices, and fissures filled by deposi- 

 tion in a similar way give rise to veins (Fig. 360). Here the filling 

 grows from the walls toward the center, and hence often has a banded 



Fig. 359. — A geode. About half natural size. (Photo, by Church.) 



appearance. By this filling of cracks and crevices, the circulating water 

 heals the breaks in the rocks. Frequently a crushed zone is thus restored 

 to a solid state. When the fissures are deep and wide and traverse 

 different formations, conditions are afforded for very complex deposits, 

 and for the concentration of rare and valuable material originally dis- 

 persed through a great mass of rock. Ore deposition in such veins is 

 usually treated as a theme by itself, but it is really but a declared expres- 

 sion of the work which the percolating waters are doing throughout all 

 the rocks which they penetrate. Most of the fine crystals that grace 

 mineralogical collections were formed in cavities and fissures by deposi- 

 tion from circulating mineralized waters. 



Solution as well as deposition. — A further phase of the process needs 



