47§ WHITMAN CROSS 



maximum possible amount of magnetite is deducted. The 

 remainder is then calculated to ioo. The analyses are not given 

 in their original form. 



The systematic plan of Schrockenstein consists, in his later 

 publication, in establishing five classes of silicate rocks, according 

 to the relations of A1 2 3 to RO (=CaO + MgO -f- FeO) as 

 shown by the percentages of the calculated remainder after 

 deducting magnetite. 

 _ RO i TT RO i- i TTT RO *^ i 



iv ^< l ->*- V -?°->I 

 ' A1 2 3 i 4' ' A1 2 3 1 



Two orders appear under each class according as lime or 

 magnesia dominates. 



Although Schrockenstein professes to use the latest infor- 

 mation, as stated in the title of his recent publication, his results 

 are based upon the discussion of 340 analyses, many of them old, 

 while on the other hand no single one of the hundreds of analyses 

 made in the laboratory of the United States Geological Survey, 

 within the past twenty years, is utilized. Hundreds of European 

 analyses of recent date are also ignored. 



It seems unnecessary to give any further details concerning 

 Schrockenstein's propositions. He is not actually treating rocks 

 and his superficial considerations of chemical composition can 

 have no bearing upon true petrographic system. 



H. 0. Lang, i8gi. — An attempted arrangement of igneous 

 rocks on a chemical basis by H. O. Lang, 1 in 1891, is founded 

 on the idea that since the feldspars are the most important con- 

 stituents of eruptive rocks, an appropriate and practical chemical 

 basis of classification may be found in the relations of the bases 

 potash, soda, and lime, the distinctive elements of the various 

 species of feldspar. In one case Lang used the percentage 



*" Versuch einer Ordnung der Eruptivgesteine nach ihrem chemischenBestande," 

 Tscher. Min. Pet. Miith., XII, 1891, p. 190. 



" Das Mengenverhaltniss von Calcium, Natrium, und Kalium als Vergleichungs- 

 punkt und Ordnungsmittel der Eruptivgesteine," Bull. Soc. Beige de Geol., 1891, V, 

 p. 123. 



