MINERAL VEINS. 731 



pyroxene, showing an alteration of pyroxenic and perhaps other rocks into soapstone, 

 by some magnesian process; and the serpentine of the region may be of the same 

 period of metamorphic change. Examples of the change of crystals and rocks to 

 soapstone or serpentine, occur in the metamorphic regions of New Jersey and Pennsyl- 

 vania; and thej T are common in other countries. Again, at Diana and other places in 

 Lewis County, N. Y., there are beds of a soft compact rock, which is sometimes worked 

 into inkstands, and resembles the agalmatolite of China; and, at one locality, crystals of 

 nephelite have been altered to this agalmatolite. These cases of the metamorphism of 

 metamorphic Archaean rocks may have taken place during the epoch of metamorphism 

 after the Lower Silurian, when the rocks of the Green Mountains were to so large an 

 extent crystallized. 



See, further, on the history of this branch of science and its processes, a Memoir by 

 Daubr^e, translated from the French by T. Egleston, and published in the Smithsonian 

 Annual Report (8vo) for 1861. ' 



5. FORMATION OF VEINS. 



1. Veins. — The general forms of veins are described and illustrated 

 on pages 108-114. They occupy either fissures or spaces opened be- 

 tween the layers of upturned or folded beds. Fissures or opened spaces 

 may result from any movement of the rocks, however slight, or from 

 whatever cause. Veins abound in all disturbed and metamorphic beds. 

 They may have great depth, extending through a series of formations, 

 or be confined to particular strata. Where a disturbance is in pro- 

 gress, the different kinds of rock will necessarily be fractured differ- 

 ently, according to their nature. Those that are unyielding or fragile 

 may be broken into numberless fragments, and these fragments widely 

 displaced : so that, when the opened spaces or fissures are filled, the 

 rock will be reticulated with irregular and seemingly faulted veins. 



The forming of veins by the opening of the layers or lamina? has 

 taken place especially in slate-rocks : auriferous quartz veins have to 

 a great extent thus originated. 



2. Methods of Filling Veins. — There are three ways of filling 

 veins: (1) by injection from below; (2) by infiltration from above; 

 (3) by infiltration from the rocks either side of the vein, or from those 

 bounding it along some portion of its course. Under the second and 

 third methods, heat is not absolutely necessary, though generally 

 required. 



First Method. — The first method — that by which trap dikes were 

 formed — is not the common one. There are cases, like that of the 

 Lake Superior region (p. 185), where metals or metallic ores are 

 directly associated with injected dikes. But it is always a question, in 

 such a case, whether the metallic ingredient was derived from the 

 same deep igneous source with the melted rock of the dike, or whether 

 it was received from the rocks of the deeper walls of the fissure during 

 the progress of its ejection. The vapors or mineral solutions pro- 

 duced at such a time often penetrate the rock adjoining the veins, 



