SYSTEMATIC PETROGRAPHY 467 



tinction, yet in first degree a geologist, prominent upon the 

 Prussian geological survey. 



The views held by Lossen were repeatedly expressed, and 

 were summed up in a discussion of principles entitled Uber die 

 Anforderungen der Geologie an die petrographische Systematic. 1 The 

 " demands " here forcibly expressed are based upon the idea that, 

 because the rock is a geological body, the geological relations of rocks 

 must be recognized as petrographical relations. Of all the geologi- 

 cal relations of rocks Lossen selected that which seemed to him 

 the most important, and claimed that that principle should be 

 used as the primary factor in petrographic classification. The 

 relation of rock masses to the earth sphere appealed to Lossen 

 as all important, and on that criterion all rocks were considered 

 as stratified or massive. Stratified rocks were defined as those 

 accumulated upon the earth's surface under the controlling influ- 

 ence of gravity, causing them to assume in some degree the 

 form of concentric strata normal to the radius of the earth. 

 The material of massive or eruptive rocks, on the other hand, 

 was viewed as having been forced from the depths directly 

 against the influence of gravity, consolidating like a casting, 

 under the control of surrounding conditions. Surface lavas, 

 spreading out in sheets, while controlled largely by gravity in 

 their formal relations, were referred by Lossen to massive rocks, 

 because possessing the resemblance to a casting from one pour- 

 ing, and not as built up by successive additions, layer upon 

 layer. It will be seen that the molten condition of magmas was 

 actually a leading factor in Lossen's idea, although the avowed 

 intention in using eruptive as an alternative for massive was to 

 express the force opposed to gravity, and not the molten state 

 which rendered them susceptible to that force. 



Lavas are not the only rock masses difficult of consistent 

 treatment under Lossen's principle. Pyroclastic tuffs were con- 

 sidered as illustrating the fact that "das Ineinandergehen zum 

 Wesen der Gesteinsnatur gehort." Lossen confessed, further, 

 that rocks of the first crust of the earth, according to the nebu- 



l Jahrbuch der k. pr. geol. Landesanstalt, 1883, p. 486. 



