HEAT — METAMORPHISM. 32 



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liiglier grades of metamorphism, moisture at a temperature that made it 

 superheated steam. In the state of steam it spreads through the rocks 

 with all the chemical energy derived from its high temperature, a destroyer 

 of cohesion, a powerful solvent, and a promoter of decompositions prepara- 

 tory to recompositions. 



The making of deposits of silica in the form of quartz or opal does not 

 require high heat, as already explained (page 135). In addition to the facts 

 there stated, it may be added that geodes of chalcedony and agate, 8 to 10 

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

 remains of hemispherical masses of coral, the exterior still showing the stars 

 of coral, while the interior is a great agate-lined cavity ; they were made by 

 the siliceous waters of the region, and it is not certain that the waters were 

 even warm. J. Arthur Phillips found crystallized quartz and chalcedony 

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

 Francisco, and at Steamboat Springs, in Xevada ; and Le Conte and Becker 

 have reported other similar facts. 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 solidification of rocks by means of silica is an 

 easy effect in the presence of hot moisture, and but little heat is necessary. 



Many experiments of recent years illustrate the efficiency of superheated 

 steam in confined spaces or under pressure. Mr. J. Jeffrys, in 1840, sub- 

 jected some feldspathic and other siliceous minerals to a current of steam 

 inside of a kiln made for vitrifying brown stone-ware, and with them a few 

 articles of the stone-ware. At a full red heat, little effect was produced ; but 

 above that of fused cast iron, there was rapid erosion, and in ten hours 

 "more than a hundredweight of mineral matter had been carried away in 

 the vapors." Daubree, having inclosed a little water in a strong glass tube 

 and subjected it to a temperature of 750° F. (400° C.) for several weeks, 

 obtained, besides a hydrated silicate allied to the zeolites, quartz in well- 

 defined crystals, and, in another case, perfect crystals of the light-colored 

 variety of pyroxene, called diopside. The glass was completely dissolved 

 and used in making the crystals. A clay, from near Cologne, used in mak- 

 ing crucibles, heated in the glass tubes, became charged with scales of a 

 mica or chlorite (the quantity being too small for an analysis). Crystals 

 of the feldspar, orthoclase, occur in the cavities of some igneous rocks in 

 the copper region of Lake Superior as a secondary product, and the accom- 

 panying facts make it certain that it was made by means of heated moist- 

 ure. But experiments in closed tubes containing the ingredients and water 

 have succeeded in making orthoclase and albite, with also tridymite (Haute- 

 feuille, Friedel and Sarrasin), while dry heat has always proved a failure. 

 Experiment has been successful in obtaining, by fusion, the feldspars, oligo- 

 clase, labradorite, and anorthite, and also the rocks containing them. 



