INTRODUCTION. 321 



patches or particles of a very acid glass a case described by G. voru Rath in a so-called trachyte from 

 Monte Amiata, in Tuscany. Such a rock can only be designated as a granular rock which is not entirely 

 holocrystalline. 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. 1 



According, therefore, to the new rule, strictly applied, we may have a granular 

 rock containing glass. In the case cited the glass is described as in isolated particles; 

 but the classification could not have been different had it appeared as a base holding 

 and cementing together the mineral grains, neither can the amount of this glass be 

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

 orthoclastic series, containing 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 be a granular rock, while, had the crystallization proceeded further 

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

 porphyry. Again, iu referring to the observed difference between diabase and gabbro 

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

 difference is only an apparent one, if the essence of the diabase structure be considered 

 as lying iu irrelative age of the feldspars and not in their form. 2 Yet this formal dif- 

 ference still exists and 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 

 purely 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 fairly be ques- 

 tioned whether all granular rocks are the result of one phase and whether ail porphy- 

 ritic rocks have required two phases of consolidation. 



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

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

 fore sharp boundary lines are arbitrary and undesirable. 



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

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

 would be the same. 



1 " Verfolgen wir in Gedanken den Act der Grauitbildung in seiiiem Verlaufe, so wird uach 

 Ausscheiduug der Erze, Apatite, Zirkone, Biotite, resp. Amphibole oder Pyroxene, und eiues Theils 

 der Feldspathe ein Stadium eintreten, wo zwischen den ausgeschiedenen, die fertige Hauptuiasse des 

 Gesteins bildendeu Gemengtheilen in unregelmiissigen Partien eingekleuimt ein sehr acides Magma 

 vorhanden ist, aus welchem sich der letzte Rest der Feldspathe tind der Quarz auszuscheideu batten. 

 Denken wir uns nun durch irgend welche Ursache an dieser Stelle den Bildungsprocess des Gesteins 

 plotzlich uuterbrochen, so wird der Rest von Mutterlauge amorph erstarren (er kouute unter Umstandeu 

 auch spbarolithisch, ja granophyrisch erstarren) und wir erhalten so ein korniges Gemenge der Granit- 

 miiicralien (mit Ausnahme des Quarzes) und tmregelniassige Brocken uud Partien eiues sehr saurcn 

 Glases bekanntlich ein Fall der uach G. vom Rath's Beschreibung bei eiuem sogenaunten Trachyt 

 vom Monte Amiata in Toscana vorliegt. Ein solches Gcstein kann nur als eiu korniges Gestein mit 

 nicht ganz holokrystalliner Ausbildung bczeichnet werden. Enthielte dagegen das Gestein uuter den 

 krystallinen Ausscheidungen auch den Quarz, eo ware es danu iiicht mehr als ein komiges, soudern als 

 ein porphyrisches zu betrachten." Op. cit., p. 15. 



2 "Wenn man das Wesen dtr Diabasstructur nicht iu der Form, sondern in dem relativen Alter 

 der Feldspathe sieht." Op. cit., p. 8. 

 MON XII 21 



