366 Schaller — Siderite and Barite from Maryland. 



The unit rhombohedrou r occurs on nearly all of the crys- 

 tals as small faces truncating the apex of the crystals. The 

 new scalenohedron y — {3251} occurs on all the crystals and 

 is a characteristic form for this locality. Though it varies in 

 size, becoming relatively wide and short or narrow and long, 

 its general form is shown in figure 1. 



The zone v y y Yl v ri is sometimes somewhat striated between 

 y and y xl , and using the dot signal, it was seen that there was 

 a maximum of brightness in the position required for the faces 

 { 7.6.13.1 } and fll§0}. It could not be shown, however, that 

 these faces were actually present. 



The formy= {0221} occurs as broad 

 dull faces giving no reflection and only an 

 almost imperceptible haze of light. Meas- 

 urements of the /o-angle gave values from 

 59° to 62°, calc. 62° 17'. 



The concavities give an indefinite blaze 

 of light in the zone of the negative rhom- 

 bohedrons, about 80° from the base, so that 

 they do not reach down to the prism zone. 

 A study of these hollows on the goni- 

 ometer, using the dot signal, showed that 

 they consisted of vertical striations approxi- 

 mating in the center of the form {0661 [, 

 and at the extreme edge to {1120}, with 

 many forms in between these. 



Fig. 1 is an attempt to illustrate the 

 actual appearance of these crystals, show- 

 ing, particularly, the concavities described. 

 The indentations in the orthographic pro- 

 jection are somewhat exaggerated. With 

 the exception of these and the broad dull 

 faces of {0221}, the faces of the crystals 

 are highly polished, and do not show any 

 etching. It seems probable that these 

 hollows should be regarded as the result 

 of an incomplete or skeletal growth rather 

 than as the result of etching. They are, 

 in a way, analogous to the hopper-shaped 

 crystals of sodium chloride, where the 

 two faces 001 and 100 (in one zone) alternate and the result- 

 ant hollow has, in cross section, a Y-shape. In the case of the 

 siderites, however, instead of an alternation of the two faces, 

 0661 and 1120, there is a gradation from the prism to the 

 rhombohedron and the result is a rounded hollow instead of a 

 sharply angular one, as in the case of sodium chloride. 



Siderite. 



