130 Washington — Plauenal Monzonose (Syenite). 



For this purpose Dr. Whitman Cross furnished me with 

 material from a specimen (No. 234) in the Petrographic Refer- 

 ence Collection of the United States Geological Survey, Prof. 

 J. F. Kemp with part of a specimen collected by himself in 

 1886 and now in the collection of Columbia University, and 

 Prof. L. Y. Pirsson with a piece of a specimen which had 

 been bought by him in 1891 of Blatz in Heidelberg, and now 

 in the collection of the Sheffield Scientific School. Each of 

 these specimens was practically identical with mine in all 

 respects, and the pieces, though not large, were ampty suffi- 

 cient to furnish representative material for analysis* It is a 

 great pleasure to express my thanks to these friends, who have 

 so kindly and generously aided the present investigation. 



As three complete analyses would have involved the expendi- 

 ture of much valuable time, it was decided instead to deter- 

 mine the alkalies alone in each of these specimens, since the 

 divergencies between Zirkel's analysis and mine were most 

 serious as regards these constituents, and it was considered 

 that any marked variation in the igneous mass would be surely 

 manifest in the figures for these. The results of the deter- 

 minations were fully confirmatory of my first complete analy- 

 sis, and indicate a remarkably uniform composition throughout 

 the mass, at least as far as the specimens extend. Not only is 

 this uniformity of interest as showing the true chemical com- 

 position of this well-known rock-type, but it is of some import- 

 ance in a more general way by indicating the uniformity 

 Avhich may obtain through large igneous masses. It may be 

 added that the alkali determinations were made in every 

 case by the Lawrence Smith, method, which experience has 

 shown to be the most expeditious and fully as accurate as, 

 if not more so than, the Bunsen method usually employed 

 abroad. Since this paper has been set up the chief constituents 

 in Cross's specimen have been determined, the results being 

 embodied in the table of analyses. 



Although the rock is well-known to all petrographers, it will 

 be briefly described in terms of the quantitative system of 

 classification, as an illustration of the methods of this and of 

 the form in which the descriptions of rock-types according to 

 this system may be stated. 



I may add that it is with reluctance that I am thus com- 

 pelled to point out and to correct an error of my teacher and 

 friend, Professor Zirkel, and can but mention the fact that the 

 analytical methods and standards of forty years ago had not 

 attained the degree of accuracy which they have at present. 



