132 Field Columbian Museum — Geology, Vol. III. 



by this peculiarity and thus the orientation of the crystal is much 

 facilitated. These striations are probably the result of oscillation of 

 the plane a (100) with prismatic planes, and in some crystals, as 

 shown in Fig. 3, PI. XLVI, this is obviously the case. 



As is usual with anglesite, the crystals exhibit a variety of habits, 

 no single habit predominating. The habits noted may be described 

 as tabular, prismatic, and pyramidal. Of these perhaps the most 

 striking and unusual to anglesite is the tabular one. It is produced 

 by a pronounced development of the basal planes uniting with a short 

 unit prism. Figs. 2 and 3, PI. XLV, showing crystals taken from 

 specimens having the Museum Numbers M 9579 and 7293, illustrate 

 the habit. Crystals of this type may be simple or highly modified. 

 In some the prism becomes longer in the direction of the vertical 

 axis, as shown in Fig. 3, PI. XLV. Crystals of this type are usually 

 attached by one of the planes of the unit prism so that their orienta- 

 tion is not always obvious at a glance. 



Prismatic habits are common and, as is usual with anglesite, the 

 habit may be produced by elongation in the direction of either of the 

 axes. Crystals elongated in the direction of the vertical axis are 

 illustrated in Fig. 4, PI. XLV (Mus. No. M 9590) and Fig. 1, PL XLVI 

 (Mus. No. M 9586). Of these No. M 9590 is from the Bullion Beck 

 mine. It exhibits the peculiarity of having the prism m (no) at 

 one end of the crystal and the prism 8 (230) at the other. As illus- 

 trated in the figures, the crystals of the vertically elongated prismatic 

 habit may have pointed or blunt terminations. The blunt termina- 

 tion is produced by broad development of the basal plane and gives an 

 especially characteristic form (Fig. 1, PI. XLVI). Crystals elongated 

 in the direction of the brachy-axis are illustrated by Fig. 2, PL XLVI 

 (Mus. No. M 9587). Crystals of this type are inclined to stoutness. 

 They are semi-transparent and have the planes well developed. 

 The finest crystal of the whole collection exhibits this habit, the elon- 

 gation in the direction of the brachy-axis being, however, less than 

 shown in the figure of the type. This crystal has a length of 2 centi- 

 meters in the direction of the vertical axis and 1 . 5 centimeters in the 

 direction of the macro-axis. It is perfectly transparent and colorless. 

 Another superb crystal of this habit has a well-marked canary yellow 

 color. It is about three-fourths the size of the one previously men- 

 tioned. 



Fig. 3, PL XLVI (Mus. No. M 9582) shows a crystal form in which 

 the elongation occurs in the direction of the macro-axis. Crystals of 

 this type are as a rule small, having a maximum length of about 1.5 



