38 CLASSIFICATION BASED ON MINERAL COMPOSITION. 



Amongst those who have acknowledged Hunt's claim are both Danas,* 

 Silhman,t Leeds, J Rutley,§ Hawes, || and Foiique and Levy. ][ 



'Edward Dana says that the theory " was offered by Hunt, and has since 

 been developed by Tschermak ; " Rutley, that Hunt's conclusions are almost 

 identical with those of Tschermak; again, James D. Dana, mistaking Hunt's 

 views, stated of the latter, '' In the view , . . with regard to the molec- 

 ular relations of the feldspars, he appears to have anticipated Tschermak by 

 ten 3'ears ; " while Silliman goes so for as to say, " Here will be found devel- 

 oped his [Hunt's] views on the constitution of the feldspars, which were 

 some 3^ears later adopted without aclvnowledgment by Tschermak." Leeds 

 appears to have been the only one who recognized the essential difference, 

 between the indefinite mechanical-mixture view adopted by Hunt, and the 

 definite molecular-union theory of Tschermak ; but he failed to see the logical 

 conclusion to be derived, — that Hunt*'* was in no sense the originator of 

 Tschermak's theory, and that all the charges of appropriation made against 

 the latter ought to be entirely withdrawn. 



In 1875, Descloizeaux, from the optical properties of the plagioclastic feld- 

 spars, concluded that andesite was an altered oligoclase, but that labradorite 

 and oligoclase are distinct species, and not isomorphous mixtures. He looked 

 upon their optical pi'operties as opposed to Tschermak's theory. ft To explain 

 the chemical composition, Descloizeaux calls attention to the theory of 

 Friedel and others, that the several feldspars differ from one another only 

 in their proportions of silica, forming a series whose common difference is 

 Si02 : e. g., anorthite -j- SiOo = labradorite ; labradorite -|- Si02 = andesite ; 

 andesite -|- SiO.j = oligoclase ; and oligoclase -j- Si02 = albite. 



While Descloizeaux admits that the composition of the feldspars may be 

 explained as w^ell by Tschermak's theory as by Friedel's, yet he holds that 

 the latter accords better with the optical and crystallographic characters of 

 the species. 



Vom Rath, on the other hand, is of the opinion that the chemical consti- 

 tution of the feldspars is most satisfactorily represented by Tschermak's 

 theory, and holds that the formation of the intermediate triclinic feldspars 



* Am. Jour. Sci., 1S75 (3), ix. 102 ; Text Book of Mineralogy, 1877, p. 297- 

 t Aiuer. Chemist, 1874, v. 106. + Amcr. Chemist, 1877, vii. 335. 



§ Tlie Study of Rocks, 1879, p. 95. |i Geol. New Hauipsliirc, 1878, iii. part 4, p. 89. 



<Jl Miner. Microg., 1879, p. 206. 



** Bull. Mus. Comp. Zool., 1S84, vii. 370-374, 445-454, 458, 459. 



ft Ann. Chiitiic Physique, 1875 (5), iv. 429-441; Coniptes Bendus, 1875, 1x.n.x. 364-371; Ncues Jahr. 

 Miu., 1875, pp. 279-281, 395 •399. 



