EARTHY MINERALS. 59 



ical and nodular masses. In its various forms it is used 

 for making various articles cameos, snuff-boxes, but- 

 tons, marbles, etc. The carnelian, which is so familiar to 

 you, is of a bright red color, sometimes yellow. There 

 is one variety of an apple-green hue, the color being given 

 to it by nickel, the metal found alloyed with iron in me- 

 teorites ( 71). In agate the colors are arranged either 

 irregularly in spots or clouds, or in regular layers around 

 a centre. Sometimes the lines are zigzag, like the lines 

 of a fortification, and then it is called fortification agate. 

 Sometimes the oxyd of iron is arranged in this mineral 

 in moss-like branches, giving it the name of moss agate. 

 In the onyx the differently colored material is in horizon- 

 tal layers, the colors being commonly a light brown and 

 a white. This is the mineral which is used for cameos, 

 the figures being cut in one layer, the other layer furnish- 

 ing the background. The variety of chalcedony called 

 cat's eye, of a greenish-gray color, has internal reflections 

 of light which resemble those of the eye of a cat. 



115. OpaL This mineral belongs to neither of the va- 

 rieties of quartz r differing from them in composition by 

 containing water, the water varying in different speci- 

 mens from 5 to 1 2 per cent. The colors of opal vary 

 much from the presence of coloring matters, which, espe- 

 cially in the dark and thoroughly opaque specimens, may 

 be regarded as impurities. The color of the noble opal, 

 so highly prized as a gem, is commonly milky, and has a 

 play of brilliant but delicate internal reflections. In the 

 fire opal these reflections are a fire-red in color. Opal 

 has not as much hardness as quartz. 



116. Silica in Solution. I have stated that silica is in- 

 soluble. It is ordinarily so. But sometimes it is dis- 

 solved in water by means of the potash in it, as explained 

 in 265, Part II. Thus, in the waters of the geysers 

 and some other hot springs, there is silica in considerable 

 quantity. Here, undoubtedly, heat aids the solution, but 

 it is not its principal cause, for silica is dissolved in water 



