334 DYNAMICAL GEOLOGY. 



4. Making of Veins. 



In the making of veins, the material has usually been deposited against 

 the walls ; and from the wall layer thus made there has been a thickening to 

 the center. The work is, therefore, centripetal. 



The materials have been introduced either (1) from above, or (2) lat- 

 erally, from the rocks adjoining some part of the fissure, or (3) from below. 

 The filling of superficial cracks is done usually without aid from heat. But 

 in most vein-making, heat has been required. 



1. Superficial Vein-making, not requiring Heat. — The shallow cracks of 

 rocks, like those of mud-beds, and any cavities opening upward, may take in 

 calcite, silica, or other ingredients from cold solutions, and make superficial 

 veins. The process is mere deposition, and commonly without heat. In 

 a similar manner cavities and caves have sometimes become filled. Or 

 when a bed is slightly calcareous, permeating waters have taken into solu- 

 tion some of the calcareous portion (calcite), and if cracks or fissures existed, 

 have filled them with calcite. Siliceous solutions, in like manner, may make 

 veins of quartz. So any solution made by oxidations or other means, may 

 carry material into cracks and produce veins or veinlets. 



2. Vein-making requiring Heat. — Vein-making requiring heat is carried 

 on in regions of hot springs in a superficial way. But in general, the process 

 has gone forward in fissures permeating hot rocks, and the work of filling 

 has been dependent on the heat and moisture the rocks afforded. These 

 fissures, in the case of the majority of veins, have not descended to regions 

 of fusion ; while in the case of other veins of even greater importance, as 

 regards ore-production, they have reached fusion-depths and have let up 

 melted rock. The veins of the first of these kinds are especially common 

 in Archaean rocks ; while those of the second belong mostly to later time. 



Superficial Vein-making. 



Superficial vein-making is in progress at hot springs in Nevada, Cali- 

 fornia, and elsewhere. Such springs, making solfatara areas, are usually 

 in regions of former eruptions. 



In Nevada, at Steamboat Springs, according to J. Arthur Phillips (1879), 

 fissures are being lined with a siliceous incrustation, while at the same time 

 steam and gases, with boiling water, are escaping; and ''they have been 

 subjected to a series of repeated widenings," and become lined, to a thick- 

 ness of several feet, with silica, which is in bands, amorphous and crystalline 

 alternating, and contains some hematite, pyrite, and chalcopyrite. Accord- 

 ing to Mr. Laur (1863), the silica of these fissures contains also traces of 

 gold ; so that the facts exemplify, as he states, the essential points in the 

 origin of auriferoiis quartz-veins. This view was presented by B. Silliman 

 and W. P. Blake, in 1864, with reference to the banded quartz-veins (gold- 

 bearing) of Bodie Mountain, north of Mono Lake, which are contact veins 

 intersecting porphyry. At Clear or Borax Lake, as observed by Mr. Phillips, 



