J. P. Iddings — Fayalite in the Yellowstone Park. 59 



Considering the Fe 2 3 as the opaque coating of alteration, 

 and the insoluble SiO„ as the adhering quartz, the composition 

 of the unaltered mineral will be: 



Oxygen ratio. 



Si0 2 25-61 32-41 17-275 1*12 



FeO 51-75 65-49 14-539 \ 



M2O 1-66 2-10 -840 » 



1-00 



79-05 



100-00 



which, is essentially the composition of the unisilicate fayalite. 

 Perfectly fresh, unaltered crystals about a millimeter long 

 were subsequently found in small lithophyses in compact ob- 

 sidian from half a mile north of Lake of the Woods. A very 

 careful qualitative test showed that the crystals were an iron 

 silicate containing no magnesia. They are in thin, square or 

 rectangular plates, of a light honey-yellow color, perfectly 

 transparent and free from inclusions of other minerals, but 

 occasionally containing gas cavities. They show very slight 

 pleochroism, pale greenish yellow parallel to the b axis and 

 golden yellow parallel to the c axis. The cleavage parallel to 

 the brachypinacoid is good, but a second at right angles to the 

 first is less distinct and is probably in the plane of the macro- 

 pinacoid as in olivine. 



1. 2. 



Mr. S. L. Penfield, of the Sheffield Scientific School, has 

 kindly determined and figured the crystallographic forms pre- 

 sented by the rectangular, tabular crystals from the locality 

 north of the Lake of the Woods, and the more elongated and 

 pointed crystals, from Obsidian Cliff. The measurements were 

 made on a thin tabular crystal 01 mm thick and 0'8 mm broad, 

 which was broken across the prismatic zone. The observed 

 forms were a (100, i-l), b (010, i-fy, s (120, i-2), e (111, 1), d 

 (101, 1-i), Jc (021, 24). Arrangement of planes quite constant 

 as in figure 1. For fundamental angles the two best reflections 

 were chosen. 



d 



s, 100 

 d, 101 



120 = 42° 31' 

 101 = 103° 17' 



giviag S, : b : c = 0-4584 : 1 : 0-5791 



