62 L. V. Pirsson — Remarkably developed Calcite Crystals. 



In fig. 1 the simplest and most common of these forms is 

 shown; v = l 3 (2131). In this figure the crystal is drawn in 

 the usual position of the plus scalenohedron ; in fig. la the 

 same crystal is shown after a revolution of 60° into the position 

 of a minus scalenohedron in order to. better present the great 

 development of four of the scalenohedron faces into a prism. 

 This is the usual development of the greater number of speci- 

 mens examined. The prism like form varies greatly in length 

 with different crystals. These twins and simple individuals 

 occur intimately associated together on the same specimen. 

 Some twins are very large, up to eight or ten inches long. 

 The larger crystals are however of the simple twinned form 

 described, while the smaller ones are in general more highly 

 modified. One of these is shown in fig. 2 in its normal posi- 

 tion and in fig. 2a revolved, as in the previous example, 60° to 

 show its prismatic development. On this crystal the forms v, 



1% (2131); F, fS (7 111 15); and/, -2 (0221) were ob- 

 served. 



The most common habit of the smaller crystals is shown, 



1 1 

 however, in fig. 3, where of the scalenohedron, \ 3 one pair 



of faces is represented, largely developed in front, cutting off 

 the main form l 3 above the re-entrant angle, while the other 

 two pairs are either wholly wanting or are developed in so 

 small a degree as to practically be so. It will be observed in 

 the study of these crystals that all scalenohedral faces which 

 appear as bevelments of the acute prism-like edge, must lie in 

 the zone 1011 /\ 0112 and this affords an important aid in their 

 identification. The crystal faces do not generally reflect light 

 well but sufficiently so for fair measurements on the reflecting 

 goniometer and the determination of the forms. 



Fig. 4 represents a very remarkable calcite from the collec- 

 A tion of Mr. Norman Spang, who very gener- 



ously presented it to Professor Penfield and 

 the latter has kindly allowed me to study and 

 figure it in connection with these Mexican 

 crystals. Mr. Spang obtained it in the south- 

 west, but the exact locality could not be dis- 

 covered. From its general similarity to the 

 described forms, it will be suspected of having 

 come from the Mexican locality. It differs 

 from all of them which have been examined 

 in being much more highly modified and 

 especially by the presence of minus forms. 

 The forms observed on this crystal were r, 1, 



(1011); v, 1% (2131); JT 9 2 3 , (2461); J -#, 



(8 20 28 9); R, -f, (1 8 12 5); x, -2 2 , (1311) ; JV -4* 

 (4 16 20 3), and X, 1% (3142). 



