454 WHITMAN CROSS 



The use of the soda-lime feldspars — oligoclase, labradorite, 

 and anorthite, as factors in the subdivision of feldspathic rocks, 

 found in the Lehrbuch, had been shown by the microscope to be 

 an error, and it disappears without comment. The mineral com- 

 position of rocks is here applied to their classification in a quali- 

 tative way almost exclusively. Zirkel remarks: "To the non- 

 feldspathic, non-schistose, composite rocks belong among others : 

 eclogite, tourmaline rock, olivine rock, eulysite and saussurite- 

 gabbro." Of these only three are described and within a com- 

 pass of two pages. 



The age distinction in classification of igneous rocks is freely 

 characterized by Zirkel as unnatural and undesirable and it is 

 not formally recognized in this work as it was in the Lehrbuch, 

 yet he could not see his way to carry out the reform necessary 

 to its rejection and retained in description many of the duplicate 

 terms based on it. 



Structure and crystalline condition were not given a defined role 

 in Zirkel's new system, but in practice the granular, porphyritic, 

 fluidal and glassy forms were distinguished. 



A. von Lasaulx, 1875. — The Elemetite der Petrographies by A. 

 von Lasaulx, issued in 1875, ' s another attempt to utilize the 

 results of microscopical study of rocks in their classification and 

 description. Von Lasaulx believed that since there are no true 

 rock species, and since transitions in all directions are most 

 common, classification must consist in the establishment of types, 

 about which should be grouped the intermediate kinds of rocks. 

 He announced as his guiding principle that rocks must be classi- 

 fied upon the basis of simple, definitely known and easily 

 recognized, morphological properties. Genetic criteria did not 

 seem to him applicable because always more or less hypothetical 

 and in some cases entirely so. He, therefore, discards the 

 primary classification of rocks on genetic principles, advocated 

 by von Cotta and others, and returns to von Leonhard's ele- 

 mentary division into Simple, Composite and Clastic rocks, omitting 

 the Apparently simple class as no longer necessary. 



1 Elemente der Petrographie, Bonn, 1875, pp. 486. 



