324 DYNAMICAL GEOLOGY. 



The lime-soda feldspars, labradorite and oligoclase, and the lime feldspar, anorthite, 

 were obtained in crystals, from the fusion together of their constituents, by Fouqufi and 

 L6vy in 1878. 



Two days of fusion only were required for labradorite. They have also produced by 

 similar methods the rocks augite-andesyte, doleryte, basalt, and others. The augite- 

 andesyte was made by fusing together 3 parts of oligoclase and 1 of augite, and it con- 

 tained octahedrons of magnetite formed at the expense of part of the augite. Doleryte 

 was obtained in like manner by substituting labradorite for oligoclase. Such experiments 

 prove that these rocks may be made from the constituents by metamorphic methods, if 

 the heat is sufficient for fusion ; and other facts leave scarcely a doubt that they may be 

 formed also at lower metamorphic temperatures. 



Hawaiian caves, made by the flowing away of the lava after it was crusted 

 over, and hot with the heat that was left by the lavas, contain long stalactites 

 of basalt, with isolated crystals of the feldspar, pyroxene, and magnetite, in 

 their cavities, as explained on page 295. In this case, there was no decom- 

 position and recomposition, but simply solution, transfer, and recrystallization. 

 The heat was that left by the passing lava ; for there was no other possible 

 source ; and it was less than that of fusion, for much had been lost by the 

 expansion of the superheated vapor as it escaped. The vapors would have 

 contained sulphur, or sulphurous acid, and perhaps other ingredients, but 

 beyond increasing solvent powers, if this, these aids had nothing to do. 



These facts are eminently instructive as to the powers of superheated 

 steam. It can do transfer work, take up labradorite, pyroxene, and mag- 

 netite at one place, and transfer and deposit them crystallized in another. 

 The facts above stated also prove that superheated steam, at a high tem- 

 perature, may produce that plastic state of a rock, which is like fusion in 

 any other way in its ability to obliterate all previous structural features, 

 and which, therefore, could make granite out of materials that otherwise would 

 have the bedding of a gneiss. 



At Birmingham, Conn., 10 miles west of New Haven, the porphyritic gneiss of the 

 region comes up, in one place, through the gneiss and mica schist, as a broad and nearly 

 vertical vein, or dike, of porphyritic granite — a rock like the bedded gneiss, except in the 

 absence of bedding, and in its vein-like position. It is plainly a result of the plastic con- 

 dition of the rock in the vicinity of a fracture. In cases of veins of fine-grained granite, 

 it is always a question to be considered whether it is not the plastic granite of a region 

 of metamorphism rather than vein-made granite, or that of eniption from a deep-seated 

 source. Vein granite is usually coarsely crystalline, and consequently irregular in its 

 grain. If the granite of a narrow dike, or vein, intersecting any rock is crystalline granu- 

 lar to its wall, the evidence is conclusive that the inclosing rock was hot, and it is almost 

 certain that the conditions when it was formed were those of metamorphism. 



Further, the pressure which is so enormous in some cases of mountain- 

 making, increases the solvent power of hot moisture, and promotes the weld- 

 ing of grains by the closeness of the contact. 



Variations in degree of heat and amount of moisture. — The metamorphic 

 effects in a region are greatly varied by differences in temperature, and in 

 amount of moisture. The region has necessarily its area of maximum heat, 



