374 Notices of Memoirs — Prof. Szaho's 



II. — On the Classification and the Cheonology of the Tertiaey 

 Eruptive Eocks of Hungary. By Prof. M. Szabo. 



(Exti-acted from the " Compte rendu stenos^raphique du Congres internationale de 

 Geologie," held in Paris, Aug. 29-31 and Sept. 2-4, 1878.) Imp. Nat. 1880. i 



niO coutribute to the solution of the great problem raised by the 

 _L study of the eruptive rocks, I have devoted myself to the 

 lithological and geological examination of the trachytic district of 

 Hungary. 



The first question to be solved was this : Is there a certain relation 

 between the minei-alogical constitution and the relative age of the 

 different trachj'tic types ? 



To determine these types, I have taken as a starting-point the 

 niineralogical association which they present, and since among their 

 constituent elements the felspars play a predominant part, I have, 

 in the first instance, endeavoured to establish a prompt and easy 

 method for the determination of the different species of this group 

 in rocks. 



One of the first results of these investigations has been to show 

 that instead of the ten series of felspars established by Tschermack, 

 one may be contented, so far as the trachytes are concerned, with 

 the four principal species generally recognized by the French school. 



One may then distinguish in these rocks four principal types each 

 characterized by the nature of its predominant felspathic element, 

 viz.: — anorthite-trachyte, lahrador -trachyte, oUgoclase-trachyte, and 

 orthose-trachyfe.^ 



To define these different types, however, more precisely, it is 

 needful to add to the designation of the felspar an enumeration of 

 the principal minerals which are associated with it in each of the 

 rocks. Thus we have : — 



1. AnortMte-pyroxene-trachyte, characterized also by the absence 

 of black mica and quartz ; amphibole is not excluded. 



2. Labrador-mica-trachyte, with amphibole, with or without quartz, 

 with or without pyroxene, and with or without garnets ; in this rock 

 the amphibole plays a dominant part. 



3. Oligoclase-mica-amphibole-trachyte. In this type the amphibole 

 decreases in amount, but quartz is never absent. 



4. Orthose-mica-'trachyte, always accompanied by a triclinic 

 felspar (generally oligoclase). Quartz is never wanting, but 

 amphibole is often absent. These different types are not fixed ; 

 transitions occur between them, resulting from the mixture of the 

 felspathic species of which it is needful to take account in a more 

 detailed description. 



To appreciate the value of this classification, let us consider the 

 relative age of the Hungarian trachytes. All geologists agree in 

 recognizing that the phase of greatest activity in trachytic eruptions 

 is met with in the Middle and Upper Miocene, and it is therefore 

 possible to determine the age of the various types of these rocks 

 in the breccias and tuffs containing characteristic fossils. 



1 Translated by Frank Rutley, F.G.S. 



* The term orthose includes orthoclase and sanidine. 



