42 CLASSIFICATION BASED ON MINEEAL COMPOSITION. 



examining the crystals produced, under proper conditions. This is simply a 

 method of making qualitative tests upon material in bulk too small to be 

 tested in the ordinary way.* 



In 1876 Professor J. Szabo published f his method of determining feld- 

 spars by means of their fusion, reactions, and coloration produced in the 

 Bunsen flame, which gave, according to him, a means for estimating the 

 percentages of alkalies, etc., in the specimen examined. 



Still a third method is from the crystals formed in blowpipe beads under 

 proper conditions. This method was invented by Mr. George H. Emerson 

 in 1863,$ and later expanded by Gnstav Rose,§ W. A. Eoss,|| and H. C. 

 Sorby.^ The results are essentially similar to Boi'icky's method, qualita- 

 tive, but can be used witli small fragments. 



The last and most important method is that of separating the feldspars 

 by means of liquids of different specific gravities. In tliis way considerable 

 material of a certain specific gravity can be obtained for chemical analysis, 

 and its nature ascertained.** All tliese methods have their defects: as, for 

 instance, the feldspars which give character to the rock are of more than 

 one species, usually ; they contain more or less glass and mineral impurities ; 

 and they are subject to alteration. These factors change their specific 

 gravity and chemical relations, and make the determination of a few ci'ystals 

 of Init limited value in fixintj the condition and character of the remaiuina; 

 feldspars. Of all the methods the specific gravity one promises the most, 

 1)ut it is not believed at present to lead to any essentially valuable results 

 in determiniug minerals like the feldspars, whose very species are so inde- 

 terminate. While the before-mentioned methods, and many others not 

 mentioned, have added greatly to the knowledge of minerals, they seem to 

 have blinded most observers to the general characters of the roclcs they were 

 studvins;. 



In the coarsely crystalUue rocks ciystals of feldspar, of sufficient size 

 for analysis, can often be obtained ; but that analysis, to be of any value, 

 must proceed on tiie supposition tliat the crystal is pure, unaltered, and 



* Arcliiv der imturwisscuscliaftliclicu Landesdurcliforscliung Bolimeus, 1877, iii- 5tli Abtli., pp. 1-SO. 



f Ueber elue neue Metliode die Feldspathe audi in Gesteiueti zu bestimmen, Budapest, 1876. 



+ Amer. Jour. Sci., 1864(2), xxxvii. 414., 415 ; Proc. Am. Acad., 1865, vi. 476-494. 

 • § Moiiatsb. Berlin Akad., 1867, pp. 129-147. 



II Cliemical News, 1868 (Aiiicr. Rcpnul), ii. 74-76, 147, 148, 157-160, 196 ; Pyrology or Pire Cbemis- 

 trj', Loudon, 1875. 



% Montli. Micro. Jour., 1869, i. 349-352. 



** Tboulet, Comptcs Rendus, 1878, Ixxxvi. 454-456 ; Bull. Min. Soc. France, 1879, p. 17 ; Church, Miu. 

 Mag., 1877, i. 237, 238; Goldscbmidt, Neues Jabr. Miii., 1881 (Beilage-Baud), pp. 179-238. 



