W. S. Bayley — Rocks of Pigeon Point, Minnesota. 59 



After a careful microscopic examination of every one of the 

 thin sections of rocks described by Irving as angite-syenites 

 and a comparison of these with the typical red rock of Pigeon 

 Point and its associated intermediate varieties, the conclusion 

 is established beyond doubt that some of the former are in 

 every respect similar to the typical red rock of the point, while 

 the others are as certainly identical in all essential particulars 

 with those varieties which have been called its intermediate 

 varieties. 



(III.) Chemical and General. 



Prom a mere microscopical examination of different sections 

 of the various phases of the red rock on Pigeon Point, one 

 would naturally be lead to regard them as portions of the same 

 magma which had crystallized under different conditions, and 

 then had undergone more or less decomposition. They both 

 possess the same mineralogical composition and present grada- 

 tion in structure from the granular to the porphyritic, with 

 granophyric groundmass. 



In order to obtain more positive evidence on this question, 

 analyses of the quartz-porphyry and also of the granular rock 

 were made by Mr. W. F. Hillebrand in the laboratory of the 

 U. S. Geological Survey, with these results : 



I. Analysis of the powder of seven of the freshest specimens 

 of the granular rock. 



II. Analysis of the powder of three of the quartz-porphyries. 



III. Analyses of the granite from Bejby, Sweden ; contain- 

 ing red orthoclase, gray and brownish gray quartz, black mica, 

 and a few flakes of a golden yellow mica.* 



I. IT. III. 



SiO„ 72-42 74-00 73-32 



TiO; -40 -34 



A1 2 3 13-04 12-04 14-25 



Fe 2 3 -68 -78 



FeO 2-49 2-61 2*60 



MnO -09 -05 -09 



CaO -66 -85 -83 



BaO -15 -12 



MgO -58 -42 



K 2 4-97 4-33 4-96 



Na 2 3-44 3-47 3-21 



Li 2 G tr. tr. 



H o 1-21 -86- 1-22 



P 2 6 -20 -06 



CI tr. tr. 



100-37 99-93 100-48 



Sp. Gr. 2-620 2-565 



* Gerhard: Neues Jahrb. f. Min., etc., 1887, ii, p. 271. 



