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THE POPULAR SCIENCE MONTHLY 



quartzite, clays become hardened and lustrous, and coals assume a 

 form like coke and graphite. Crystalline minerals and gems are 

 formed, the rarest ones under a combination of powerful forces of 

 heat and pressure which has been imitated by man only in the feeblest 

 degree, most notably in the production of minute artificial diamonds by 

 Mr. Hannay. The steam-jets which issue from volcanic fissures carry 

 up fragments of rock torn from the sides of the vent, in the cavities 

 of which beautiful crystallized products are often found. The various 

 metallic minerals have nearly all been brought from away down in 

 the earth's crust and deposited upon the sides of rock-fissures, the 

 same volcanic forces opening the cracks through the solid rock, and 

 then bringing up the metallic compounds and causing them to crystal- 

 lize on the sides of the fissures. The cavities of the igneous rocks, 

 when filled with water, constitute laboratories in which real chemical 

 reactions take place where the materials of the lava are gradually 

 dissolved and recrystallized in new combinations, and the agates, the 

 onyxes, the rock-crystals, the Iceland spars, and the class of zeolites 

 have been formed. " No one can visit a large collection of crystalline 

 minerals without being struck with the great number of beautiful sub- 

 stances which have thus been formed as secondary products from vol- 

 canic materials." 



Hot springs, geysers, and carbonic-acid springs, which are also vol- 

 canic in their origin, afford another variety of curious products. Hot 

 springs often contain in solution large quantities of silica, which has 

 been taken up at the moment of its separation from the alkalies or 

 alkaline earths with which it has been combined, and which is deposited 

 when the water, having reached the surface, is relieved from pressure 

 and cools down. Thus are formed the basins of the geysers of Iceland, 



i'n;. 0. Sintfr-Oones, surrounding the Orifices of CIiysers. 1. Basin ci the Great Geyser, 

 Iceland. 2. Hot-Fprinsr cone. 3. Old Faithful. 4. Tin- Giant Geyser. 5. Liberty-Cap. (2, 8, 

 4, and 5 are in the Yellowstone Park district of the Rocky Mountains.) 



and the curious structures with fanciful names which distinguish the 

 geysers of the Yellowstone. The outlines of some of these structures 

 are given in the figure (Fig. 9). The deposited silica is known to 

 geologists as " sinter." Carbonic-acid water, in a similar manner, 



