94 FIELD GEOLOGY. 



Metallic, like the ordinary lustre of metals 



Sub-metallic 



Adamantine, like that of a diamond. 

 Hardness and Effervescence. The tests by which, 

 in the field, the nearest approach can be made to the de- 

 sired result are its "hardness" and "effervescence." 

 The former is roughly determined by means of the steel 

 blade of a pocket-knife, the latter by application of 

 dilute nitric, sulphuric, or hydro-chloric acid (usually 

 about one part acid to five water). A rock or mineral 

 soft as Talc or Bock-salt may be scratched by the finger- 

 nail ; between those and Apatite, by the knife with ease ; 

 be} r ond that, and including Orthoclase Felspar, with 

 some degree of force ; while those of greater hardness 

 are not marked at all by the steel. A drop of dilute 

 acid applied to the fresh fracture will cause effervescence 

 rapid, if the specimen consist of pure carbonate (of lime 

 or otherwise) ; slow, if only partly composed of carbonate , 

 and none whatever if it be a sulphate or a silicate. The 

 following table is easily remembered, and applies in 

 nearly all ordinary cases. The exceptions are not nu- 

 merous, and they present well-marked characteristics of 

 their own ; in such cases further tests if necessary must 

 be applied at home. In practice, so many slight varia- 

 tions are found, that they are incapable of simple classi- 

 fication ; but then judgment must guide the observer, 

 and an acquaintance with cabinet specimens will be 

 found of great service, surpassed only by actual field ex- 

 perience. 



If a rock or mineral : 



Scratch with ease and effervesce freely, it consists of 



Calcite or Limestone. 



