INTRODUCTION. 321 



patches or iiiiilicles of a very acid glass — a case described by G. vom Rath in a so-called trachyte from 

 Moute Aniiata, iu Tuscany. Such a rock cau ouly be designated as a granular rock which is not entirely 

 bolocrystalliue. On the other hand, if the rock contains quartz among its crystalline particles, then it 

 may no longer be regarded as granular, but rather as a porphyritic rock.' 



According, therefore, to the new rule, strictlj' applied, we may have a grauuhir 

 rock contaiuiug glass. Iu the case cited the glass is described as iu isolated particles ; 

 but the classificatiou could not have been diffenut had it appeared as a base holdiug 

 jiud ceuientiug together the miueral grains, neither can the amount of this glass be 

 restricted under the cousideratious which gave rise to the definition. A rock of the 

 orthoclastic series, contaiuiug crystals of ore, biotite, apatite, plagioclase, and some 

 orthoclase, imbedded in glass or microfelsite, which might compose more than half of 

 the mass, would still he ii granular rock, while. Lad the crystallization proceeded further 

 aud some quartz been added to the other minerals, the product would have been a 

 porphyry. Again, iu referring to the observed diflereuce between diabase aud gabbro 

 resulting from the formal development of the feldspars, Roseubusch remarks that this 

 difference is ouly an apparent on"e, if the essence of the diabase structure be considered 

 as lying in the relative age of the feldspars and not iu their /o/wj.^ Yet this formal dif- 

 ference still exists aud must be described; but, if Roseubusch's definitions be adopted, 

 it cannot be described as structure. 



These instances have been considered somewhat in detail, to show clearly the cor- 

 rectness of the statement that Roseubusch desires to replace the structural groups by 

 l)urely genetic ones, and also to show that the two divisions are not coincident in 

 extent. In regard to the latter point it seems to the writer that it may f;iirly be ques- 

 tioned whether all granular rocks arc the result of one phase aud whether ail porphy- 

 ritic rocks have required two phases of consolidation. 



Finally, the great precision aimed at by Professor Roseubusch iu his new defi- 

 nitions seems to be unnatural. Rock groups bleud insensibly in all directions; there- 

 fore sharp boundary lines are arbitrary and undesirable. 



In the following rock descriptions the terms "granular" aud "porphyritic" are 

 used iu the purely structural sense. Were the genetic principle applied the grouping 

 would be the same. 



' " Verfolgen wir in Gedanken den Act der Granitbilduug iu seiuem Verlaufe, so wird nach 

 Ausschciduug der Erze, Apatite, Zirkone, Biotite, resp. Amphibole oder Pyroxene, und eiues TUeils 

 der Feldspathe cin Stadium eintreten, wo zwischen den ausgeschiedenen, die fertige Hauptniasse des 

 Gesteins bildendeu Gemengthcilen in unregelmiissigen Partien eiugeklemmt eiu sehr acides Magma 

 vorhandeu ist, ans welchem sich der letzte Rest der Feldspathe uud der Quarz auszuschei'deu hiitten. 

 Deuken wir uns nun dnrch irgend welche Ursache an dieser Stelle den Bildungsprocess dea Ge.steins 

 plotzlich nuterbrochen, so wird der Rest you Mutterlauge amorph erstarren (er kouute unter Umstiiiiden 

 aucU .spharolithisch, ja grauophyrisch erstarren) uud wir erhalteu so eiu korniges Gemenge der Granit- 

 niiueralien (mit Ausnahme des Quarzes) und uuregelmiissige Brocken uud Partien eiues sehr sauren 

 Gla.ses — bekauntlich eiu Fall der nach G. vom Rath's Beschreibnug bei cinem sogenannten Trachyt 

 vom Moute Amiata iu Toscaua vorliegt. Ein solches Gesteiu kaun uur als eiu korniges Gestein niit 

 nicbt ganz holokrystalliuer Ausbildung bezeichnet werden. — Enthielte dagegeu das Gestein unter den 

 krystallinen Ansscheidungen auch deu Quarz, so wiire es danu nicbt mehr als eiu korniges, soudern als 

 ein porphyrisches zu betrachten." Op. cit., \i. 15. 



'"'Wenn man das Wesen der Diabasstructur nicht iu der Form, sonderu iu dem relativeu Alter 

 <ler Feldspathe sieht." Op. cit., p. 8. 

 MON XII 21 



