468 WHITMAN CROSS 



lar hypothesis, would necessarily be separated from his eruptive 

 class, but he considered this point immaterial because he doubted 

 whether any such rocks were known. Metamorphic rocks are of 

 such diverse origin and present such difficulties to the system- 

 atist that Lossen considered it inadvisable to treat them as a 

 distinct class. 



In this discussion by Lossen, as in the majority of those ema- 

 nating from geologists, no appropriate distinction is made 

 between the rock mass as a formal unit and the material within 

 it, the rock proper. The primary division of Lossen is one of 

 rock masses, not of rocks. The consideration of the form and 

 position of these masses with regard to the controlling influence 

 of gravity or the opposing eruptive force, leads to only one of 

 many ways in which the geologist must classify rock bodies. 

 Other elemental subdivisions of the same bodies are necessary. 

 The geologist is, indeed, obliged to make and discuss all such 

 fundamental distinctions. The petrographer, on the other hand, 

 should not only be at liberty to select, but, in order to secure 

 logical excellence for his system, must choose, as his primary 

 principle, that one most closely connected with the factors which 

 he has adopted for use in the further construction of the system- 

 atic arrangement of rocks. It is clear that all igneous rocks, 

 those produced by the consolidation of molten magmas, possess, 

 from this origin, material properties most useful in their detailed 

 classification. If arrangement by a certain characteristic due to 

 this origin is desired, it matters not where the rocks occur in the 

 earth. They may belong to the primeval crust, or form injected 

 masses of whatever size, shape, or attitude, or appear on the sur- 

 face in lava streams. If it was mode of origin, not formal rela- 

 tions to the earth, which gave igneous rocks the common 

 characters used in their systematic arrangement, then mode of 

 origin is logically the principle to be used by the petrographer 

 to bring them into one grand division. 



H. Rosenbusch, i88y. — As has been mentioned, the force of 

 Lossen's claim, as made in earlier publications, was admitted by 

 H. Rosenbusch, in the first edition of his Mikroskopische Physio- 



