Vol. 5] 



Eakle. — Calcite from Terlingua. 



93 



rare form -\- 2R to be present. This form has only been observed 

 before by Palache, on the Lake Superior caleites. Some of the 

 crystals have a, still steeper form in this zone, perhaps + 4R, but 

 no readings could be obtained to substantiate it. 



Negative rhombohedrons. — The predominating form on the 

 crystals is — 2R. The faces are very bright and permitted of 

 accurate polar orientation of the crystals. The rhombohedron 

 — Vs'R which is so common for calcite is practically wanting on 

 these crystals. It was only observed once, and then as a mere 

 line face on the edge formed by the two faces of the sealenohe- 

 dron %R%. The steep form — 11R occurs on all the crystals 

 in very narrow faces. It occasionally grades into still steeper 

 rhombohedrons, only one of which, — 14R, could be definitely 

 established. 



Positive scalenohedro us. — All of the scalenohedrons are in 

 the positive sectant and are characteristically different from the 

 rhombohedrons in having either striated, dull, or etched faces. 

 The forms %R3 and %R% are present on all the crystals, while 

 the form %R%, which lies between them, occurs seldom. The 

 faces of these scalenohedrons are characteristically striated, the 

 fine lines being parallel to the zonal edge with the unit rhom- 

 bohedron. The scalenohedrons lying below the unit rhombohe- 

 dron are not as perfectly developed as the other forms on the 

 crystals. They tend to grade into one another with no sharp 

 boundary lines between and readings could be obtained for sev- 

 eral doubtful forms, but only those which could be definitely 

 established are included in this description. The forms R 1 ?!.:., 

 R%, R%, and R% are all very narrow, and of these R% is the com- 

 monest. A measurement was obtained which denoted the possi- 

 bility of R%, but as this would be a new form and could not be 

 verified on other crystals it is omitted. 



The common scalenohedrons R 3 and R 5 are present on all the 

 crystals, and between them a series of forms occur of which R x % 

 and R L % were established. R x % which has only been observed on 

 the Lake Superior caleites is a doubtful form on these crystals. 

 The faces of these scalenohedrons are generally cross-sected by 

 fine parallel striations, but these did not prevent good measure- 

 ments. The rare form 2R% is present on all the crystals, but 



