MINERAL VEINS AND ORE-DEPOSITS. 775 



veins, some of them the richest in ores, intersect non-metamorphic 

 rocks, and many are of Tertiary age. 



5. Filling of Fissures ; Making Veins. — Fissures have been filled, 

 either (1) gradually, without eruptive aid ; (2) abruptly, by means 

 of plastic rock during a time of metamorphism ; or, (3) through the 

 agency of igneous eruptions. The first are veins of infiltration ; the 

 second, dike-like veins ; the third, contact-veins. 



1. Gradually-formed Veins, or Veins of Infiltration or Segregation. — 

 Whenever a fissure is opened in a rock to a great depth the space thus 

 made is almost like a vacuum as to pressure, compared with the region 

 along side. Consequently, any materials in the adjacent rock that are 

 vaporizable, under the new conditions, or can be taken up by heated 

 moisture present, or by other solvents, will rush toward the opened 

 space. Thus depositions against the walls will be commenced ; and new 

 supplies may be kept up until the opened space is filled or the supply 

 gives out. The different materials may come from different levels in 

 the walls of the fissures, especially when they descend to depths of 

 high temperature ; and these materials may be deposited at various 

 heights along the walls above, according to the temperature and other 

 conditions which their deposition may require. The material filling 

 the fissure may hence come from the country-rock adjoining the part 

 in which it is deposited ; or from depths much below it ; but not, it is 

 believed, from areas of fused rocks or ores at the lower extremity of 

 the fissure, as in making dikes. 



In this way granite and quartz veins, ore-bearing or not, have been 

 made ; and the process has been carried forward most extensively in 

 times, as just stated, of metamorphism, since these were times also of 

 fracturings of the rocks, and of that temperature within them which 

 is required both for metamorphism and for vein-making. 



In the case of the larger granite veins, the temperature of the 

 rocks and superheated steam was probably somewhere between 1,000° 

 F. and 1,500° F., — high enough, and the action slow enough, to form, 

 as constituents of the vein, in some cases, single masses of crystallized 

 feldspar and quartz weighing tons ; crystals of mica a yard across, and 

 of beryl as large as a flour-barrel, besides making smaller crystalliza- 

 tions of rare columbates, tantalates, and other mineral species ; and 

 the enclosing rocks were changed at the time to gneiss, coarse mica 

 schists, or others equally crystalline. With less heat, hardly enough 

 for taking up feldspar material, the moisture within the rock became 

 simply siliceous, and hence made quartz veins ; and the rocks meta- 

 morphosed under the lower degree of heat, and containing the quartz 

 veins, are mostly those that bear evidence of the less heat in their 

 texture, such as fine-grained mica schist, hydromica schist, chlorite 

 schist, argillyte, and semi-crystalline limestones. 



