336 DYNAMICAL GEOLOGY. 



heat, were the source of the larger part of the fissures, and the origin of 

 their great diversity in form and positions. The heat varied, therefore, 

 from 212° F. and below, to, in extreme cases, the temperature nearly of 

 fusion ; and it slowly declined as the epoch of metamorphism closed, thus 

 making the same region to pass through conditions of high-grade heat and 

 low-grade heat, and, therefore, through conditions for different sorts of 

 veins. All the transfer and transformation processes through superheated 

 steam engaged in metamorphism were at work in vein-making with like 

 efficiency — those of low heat for filling fissures with quartz, and those of 

 higher for making feldspathic or coarse granitic veins, and other kinds. 

 Moreover, the heat so derived continued long, and disappeared with extreme 

 slowness ; so that the filling of veins was usually slow, and the crystalliza- 

 tions going on had almost indefinite time for growing, and generally became 

 coarse. The gigantic crystals of beryl, mica, and other species mentioned 

 on page 331 were thus made. 



With the heat so widely diffused, it was not necessary that the opened 

 spaces for veins should be continuous. An interrupted series of openings in 

 the upturned strata, as well as the spaces between the leaves of slates and the 

 thinner schists, would have become as readily filled by materials supplied from 

 the rocks, as they would if they had been united along continuous fissures. 



The hot-vapor solutions, everywhere at work, would have varied their 

 results according to the temperature, the moisture, and the kinds and con- 

 tents of adjoining rocks. If the fissures penetrated rocks having veins or 

 deposits of ore, or sparsely disseminated ores, the ores would be as readily 

 transferred to the veins as the stony minerals ; and the hot vapors, widely 

 distributed, might gather them in from a wide region either side of the 

 fissure, whether at its lowest or highest depths. The vapors, being under great 

 pressure, would find the fissures escape-ways, and the transfer of material 

 would therefore begin as soon as they were opened. Veins of lead ore 

 (galena), copper ores, tin ore, and other kinds are common in the same rocks 

 that elsewhere have their granite veins. Moreover, veins would be likely to 

 contain ores at their intersections with some of the rocks they cross when 

 not at other intersections. As gold occurs commonly in quartz veins, and in 

 those of the feebly crystalline schists, as chlorite schist and hydromica schist, 

 no great amount of heat was required for their formation, and the rocks near 

 by or below must have afforded the gold. 



Igneous rocks often have fissures intersecting them (due to contraction on 

 cooling, or to subterranean action) and cavities (amygdaloidal) within them, 

 that were filled, in vein-like style, from materials brought in laterally, and 

 mostly while the rock was slowly cooling, as explained on page 298. The 

 permeating hot moisture takes silica, alumina, soda, and lime from the feldspar 

 of the rock, and makes zeolites (hydrous silicates, related to the feldspar) in 

 the fissures and cavities ; and takes silica, lime, magnesia, and iron from the 

 pyroxene to make, with some alumina, the dark green chlorite ; and sets free 

 the excess of silica for making quartz crystals. 



