K Ichikawa — Some Notes on Jaj^anese Minerals. 65 



tions. Figs. 6 and 7 show the relation between the outlines of 

 the different pits and the edges of the dodecahedron {d) ; the 

 parallel lines in the lignre give the zonal structure of the crys- 

 tals. The former are observed abundantly but the latter very 

 rarely. Figs. 8 to 11 show the relation between the outlines of 

 the various forms of natural pits of very rare occurrence and 

 the edges of the icositetrahedron (?i) ; the parallel lines in the 

 figures show the zonal structure of the crystal. Fig. 12 shows 

 a single rectangular pyramidal pit ; also a group of these as 

 commonly observed on the cubic faces. 



By the above study, it is proved that in the direction of the 

 three principal axes of the crystal, elevations of an octagonal 

 pyramid are formed ; in those of the four trigonal inter-axes, 

 pits of a hexagonal pyramid are formed ; also that the edges 

 through the three principal planes of symmetry are rounded or 

 grooved. The resulting form of the etching is supposed to be 

 a hexoctahedron. 



YI. Elongated Gypsum Crystals. 



In 1909 I visited Udo, Usagi-mura, Hikawa-gun, Izumo 

 Province, and observed gypsum crystals of unusual length ; 

 notes on this gypsum and its use have already been published 

 in Japanese.* The following is an abridged translation with 

 only slight changes. The gypsum of Udo occurs associated 

 with pyrite in a clay in massive (radial-fibrous) and lamellar 

 (parallel-fibrous) forms. The massive gypsum appears as spots 

 in a hard clay and the lamellar forms in a very soft clay easily 

 pierced by the fingers ; the elongated crystals were collected 

 from the latter. Crystal faces observed are the oo i^, oo i^ do, and 

 CO i^^ , etc. The prism is elongated in the direction of the axis 

 G ; individuals measure 5 to 25'""' on the axis h, and 60 to 

 200""" on the axis c. The crystals are colorless and trans- 

 parent. Inclusions are observed of powdery gypsum, pyrite, 

 diatoms {Cyclotella^ etc.), etc. ; the diatoms were studied with 

 a magnification of 350 to 650 diameters. 



Fig. 1 (see II) shows a familiar swallow-tail twin ; A is a front 

 view and B a horizontal projection on the vertical axis of A. 

 Fig. 2 shows a specimen in which at one end the same twin 

 separates into three individual. 



Fig. 3 is a regularly developed twin. A is a front view ; B 

 a horizontal projection on the vertical axis ; C a cleavage frag- 

 ment. 



Fig. 4 is a hoop of a flat and long swallow-tail twin, arti- 

 ficially bent along the face ocP o. \ A is a front view. If the 

 hoop in A is bent in the direction opposite along the face 

 CO P^j a hoop like B is the result; in this case thin polished 



* See my notes in Jour. Geol. Tokvo, vol. xv, p. 509, 1908 ; vol. xvi, p. 92, 

 1909. 



Am. Jour. Sci.— Fourth Series, Vol. XLIV, No. 259.— July, 1917. 

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