twinning striations on the hrachyjpinacoid. 393 



which is not polished, with very distinct striations running prac- 

 tically parallel to the edge between the base and '■%-%. Were it 

 not for the play of colors on i-i, the striated brachy-pinacoid 

 could not be told at a glance from the striated basal plane. 

 How common such specimens are we cannot say; in the stu- 

 dent's collection in the Boston Institute of Technology there 

 are several specimens, probably all from one block, of exactly 

 the same character. An analysis of this feldspar by Penrield 



is given below : 



Ratio. Albite. Anorthite. 



Si0 2 54-75 -912 '528 (6) + -384 (2) 



FeA :::::: 21 '-lt -III \ w -° 88 w- 1 " 185 (°' 9V ) 



CaO l 10-60 '189 -189(0-98) 



& c ':::::. "S 21}™ ™>w 



Ign -56 



100-02 

 Sp. gr. 2-684 



The ratio of anorthite to albite = 1 : 0'93 ; per cent of anor- 

 thite, 52'45. Rhombic section ± 0, extinction angle on i-% = 

 — 19°. In this example the rhombic section, being parallel with 

 the base, agrees very closely with the above table from Tscher- 

 mak, where in andesine with AbjAn^ the rhombic section has 

 the direction — 2° and the extinction angle —16°. 



The above analytical results give, therefore, a very satisfac- 

 tory agreement both as regards the direction of the rhombic 

 section and the extinction angle on i-i with the tables given in 

 Tschermak's Mineralogy. One other specimen in Professor 

 Brush's collection (No. 1712), labeled anorthite from Mt. 

 Somma, contains a fractured crystal with striations on i-i in- 

 clined — 13°. Although no material could be obtained for 

 analysis, it is safe to assume that an analysis would show no, or 

 only a very slight, percentage of Na 2 0. 



The variations in the angles of the plagioclase crystals is con- 

 siderable, and, as vom Rath has shown, they affect the direction 

 of the rhombic section very perceptibly ; however, the change 

 in position of the rhombic section from +22° to —18 is so 

 great that a careful consideration of the direction of the stria- 

 tions will enable one to decide within very narrow limits what 

 position any plagioclase will hold in the albite-anorthite series. 



In closing, we wish to express our thanks to Professor Geo. 

 J. Brush for his liberality in allowing us to use the material 

 from his collection for carrying out this investigation. 



Mineralogical Laboratory, Sheffield Scientific School, June, 188*7. 



