OCR Text |
Show Origin of Onyx. When waters charged with carbonate carbon-ate of calcium derived from ltmcstono aro allowed to evaporato thoy deposit their load in tho form ot sinter, or tufa. This process can bo observed nt many thermal and "petrifying" springs and also In tho formation ot stalactites and stalagmites In llmestono caverns. In this way largo masses of compact carbonato are formed, somo of them ot great beauty. Tho so-called "onyx marbles," of which tho Mexican "onyx" Is a familiar examplo, aro formed In this way. Somo rock of this class Is stalagmltlc, In caverns, and somo ot It Is formed by springs. Its variations In color and toxturo, to which Its ornamental orna-mental character is largely duo, aro commonly produced by Impurities or Inclusions such as oxide of iron, or even mud and clay. |