760 » DYNAMICAL GEOLOGY. 



common feldspar, or orthoclase, occur in the cavities of some igneous 

 rocks in the copper region of Lake Superior as a secondary product, 

 and the accompanying facts make it certain that it was made by means 

 of heated moisture. Quartz, feldspar, and mica, are the common min- 

 erals of several of the most abundaut of metamorphic rocks ; and 

 pyroxene is a constituent of other kinds. Daubree's important ob- 

 servations on the minerals made in various hot springs are mentioned 

 beyond in connection with the remarks on the origin of veins. 



The making of deposits of silica does not require high heat, as 

 already explained on page 708. In addition to the facts there stated, 

 it may be added that geodes of chalcedony and agate, eight to ten 

 inches in diameter and of modern origin, come from Florida, that are 

 the remains of hemispherical masses of coral, the exterior still show- 

 ing the stars of coral, while the interior is a great agate-lined cav- 

 ity ; they were made by the siliceous waters of the warm springs of 

 the region. J. Arthur Phillips found crystallized quartz and chalce- 

 dony among the recent deposits of Borax Lake, in Lake County, north 

 of San Francisco, and at Steamboat Springs, in Nevada. Daubree 

 detected quartz in the form of chalcedony among the deposits of the 

 hot waters of Plombieres. It should be considered, further, that the 

 quartz which makes the flint and chert of the world, and has silicified 

 the fossils of many strata, was dissolved by cold waters ; it was mostly 

 in the opal state when dissolved, but was deposited in the state of 

 quartz. 



Thus the making of quartz, and quartz crystals, and the solidifica- 

 tion of rocks by means of quartz, are easy effects in the presence of 

 hot moisture ; and the crystallizing of sedimentary beds into granite, 

 gneiss, and mica schist, are other results which with its aid may be ac- 

 complished. 



With superheated steam, at a high temperature, that plastic state, as 

 experiment has shown, may be produced, which is like fusion from 

 heat alone in its ability to obliterate all previous structural features, 

 and which therefore would make a granite out of materials that other- 

 wise would have the bedding of a gneiss ; while with low heat, the 

 bedding would be retained, as in the schistose metamorphic rocks. 



With still lower temperature, solidification would be the only prom- 

 inent result, or a change of color in the rock, or an expelling of a 

 vaporizable ingredient, as in the case of mineral coal and carbona- 

 ceous schists. 



With regard to limestone, the experiment of Hall, early in the 

 century, showed that chalk, when heated in a closed tube, became crys- 

 talline calcite, with no loss of its carbonic acid. But as limestone 

 (calcium carbonate) is decomposed by hot siliceous solutions, a stratum 

 may lose part of its thickness in the process of metamorphism. 



