Washington — Plauenal Monzonose (Syenite). 129 



Art. XIII. — The Plauenal Monzonose (Syenite) of the 

 Plauenscher Griind ; by Henry S. Washington.* 



Of the group of syenites, composed essentially of orthoclase 

 and hornblende, biotite or angite, that of the Plauenscher Grand 

 near Dresden has been the longest known and is regarded as 

 one of the most typical, + especially of the hornblende-syenites 

 or syenites proper. It has also been regarded as one of the 

 few known highly potassic representatives of these rocks, or 

 those in which the amount of potash far surpasses that of soda.;}; 



In the course of a study of the highly potassic volcanic rocks 

 of Italy the close similarity of the well-known analysis by Zir- 

 kel of the Plauen syenite to those of some of the Italian lavas 

 was noted, a similarity so great that the former might be con- 

 sidered as a plutonic representative of the same magma as that 

 which furnished the lavas farther south. 



As the older analysis was not altogether satisfactory because 

 of the high summation, and its incompleteness, the oxides of 

 iron not having been separated nor Ti0 2 and P 2 5 deter- 

 mined, as well as on account of its early date, a new analysis 

 of the rock was undertaken. The material used for this was 

 a specimen collected by myself in 1897 at one of the large 

 quarries on the left bank of the Weisseritz south of the Gasan- 

 stalt. The results of this were quite unexpected, diverging 

 very much from the older one in some particulars, most espe- 

 cially in the relations of the alkalies, as will be seen later. 

 Some hesitation was felt about publishing the new analysis, 

 the more so because I was informed by Professor Zirkel, to 

 whom the analysis was shown in 1901, that the mass of syenite 

 varied considerably from place to place, a fact which might 

 explain the discrepancies. 



It was finally decided to test this last point, and to determine 

 the true composition of the rock, by examining specimens 

 from different parts of the mass. As a second visit was 

 impracticable, the object was accomplished by obtaining speci- 

 mens from different sources and collected at different times. 

 These would presumably be derived from different parts of 

 the mass and should reveal any differences in composition 

 should such exist, or, on the other hand, by their uniformity 

 show that the variations were not as great as had been sup- 

 posed. 



* The results in this paper form part of an investigation conducted for 

 the Carnegie Institution of Washington, the Trustees of whom I must thank 

 for permission to publish it here. 



+ Cf . Zirkel, Lehrbuch. II, 1894, p. 300; Eosenbusch, Mikr. Phys., II, 

 1896. p. 120. 



% Brogger, Eruptivgest. Krist. Geb., II, 1895, p. 30. 



