320 GEOLOGY AND MINING INDUSTRY OP LEADVILLE, 



not. All consider granite as a typical granular rock; and that rock which would be 

 cited by any one as typical of the porphyritic structure could scarcely be placed else- 

 where under any existing definition. This latter assertion is at least true now that 

 Rosenbusch has withdrawn his earlier definition,' by which the presence of some 

 amorphous matter in the groundmass was made essential to a porphyry. The new 

 ground taken by Rosenbusch^ in regard to the essential difference between the gran- 

 ular and porjihyritic modifications of eruptive rocks seems to the writer open to spme 

 serious objections, although the great value of many of the points so clearly presented 

 must be gratefully acknowledged by all. While a full discussion of the question can- 

 not be entered upon in this place, the chief objections to the new definition may be 

 briefly stated. 



If the writer correctly understands the position taken by Rosenbusch in his essay 

 upon the essence of the granular and porphyritic rock structures, the latter wishes so to 

 I'edefine the terms "granular" and "porphyritic" that they shall henceforth indicate 

 genetic and not structural relations. It is claimed that the typical structures hitherto 

 designated by these terms have their origin in the history of each individual rock 

 mass; the gianular rocks having come to complete solidification in the course of what 

 may be termed a single phase; the porphyritic types, on the other hand, having passed 

 through two phases, in the second of which the groundmass was formed — the matrix 

 for the crystals of the earlier phase. The genetic groups thus outlined are to replace 

 the structural ones, while the terminology is to remain the same. 



The first objection to be raised is that a new division of eruptive rocks accord- 

 ing to a genetic principle does not in any way destroy the purelj^ structural groups 

 already existing, even if the divisions produced by the two principles are exactly 

 coincident in extent. It will still be desirable and necessary to refer to rock struct- 

 ures independently of genetic connections, and the terminology of the science is not 

 simplified but rather complicated by the application of a given term in two distinct 

 senses. Granular cannot be logically used with a genetic meaning while, at the same 

 time, it is desirable to apply it in accordance with existing usage as a purely struct- 

 ural term. In the second place, it seems a matter for debate as to whether the groups 

 formed on the new principle are coincident with the structural ones. If not, we surely 

 cannot cover them by a single definition, nor use the same terms in their description. 



That the new definitions, when logically applied, do produce divisions widely 

 different from the corresponding structural groups is well illustrated in the case 

 brought up by Rosenbusch himself, in a passage of which the following is a free trans- 

 lation : 



If we follow iu thought the process of granite formation, we reach at length a i)oiut, after the 

 separation of ore-grains, apatite, zircon, biotite, hornblende, or angite, and a part of the feldspars, 

 where, between the ready-formed mineral particles which are to make up the mass of the rock, a very 

 fluid, acid residue remains, out of which some feldspar and quartz are yet to be formed. If now, through 

 any cause, the solidification of the rock be suddenly interrupted at this point, the residue will solidify 

 as amorphous substance (it might under certain conditions be spherulitic or even granophyritic) and 

 we have thus a granular mixture of the granite minerals (with the exception of quartz) and irregular 



'Fhysiograi)hie der massigeu Gesteine, pp. 86,67. 



^ " Ueber das Wesen der kornigen nnd porphyrischeu Strnctur bei Masseugesteinen." Neues 

 Jahrbuch fiir Mineralogie, etc., II, 1, It>8^. 



