Roaers. | Minerals of the Galena-Joplin District. 465 
the latter being much curved and convex. As is the case 
with many other crystals, the curved faces are due to sub- 
individuals in approximately parallel position. Figure 19 
represents the ideal crystal of this type. 
Typr 2. Forms present same as those on last type, but the 
crystals are smaller, and the m faces curved concave instead 
of convex. 
Typr 3. Small crystals with the forms p, m, r, and z, the 
latter being much striated and in oscillatory combination. 
Twins are common, the twinning plane being the ordinary 
one for marcasite,m{110}. Crystals of this type are rep- 
resented by figure 20. 
Typr 4. Cyclic fivelings (twinning plane m) occur in a 
matrix of carbonaceous shale on the Mastin ground, Galena. 
The forms are several brachydomes in oscillatory combina- 
tion, and q, which occurs as a very narrow face. Figure 24 
gives an idea of this type. Figure 25 is the detail of a part 
of a crystal, showing repeated twinning. Figures 21, 22 and 
23 are representations of other crystals of marcasite. Figure 
23 is a fine example of lamellar twinning. 
_ Marcasite in parallel position with pyrite has been observed 
at the Nickel Plate mine, Aurora. The marcasite exhibits the 
following forms: P, m, r, and z, and is partially enclosed by 
pyrite with these forms: c, p and an undetermined diploid. 
The basal pinacoid and one of the faces of the unit prism of 
the marcasite are parallel to cube faces of the pyrite. This 
parallel growth is like one described by Luedecke.’ 
Marcasite alters to limonite and melanterite. Pseudo- 
morphs of limonite after marcasite are common. — 
Quartz. 
Composition, silicon dioxide, SiOz; silica =46-7 %, oxygen = 53-3 %; hard- 
ness, 7; gravity, 2-658; color, colorless when pure; crystallization, rhombo- 
hedral system. 
Observed forms: b{1010}oR; rj{1011}4R; o{0111}—R. 
The mineral quartz is only occasionally found as crystals, 
and then usually of very small size. However, at Granby 
9. Ann. d. Phys. u. Chem. Erg., vol. 8, pp. 625-664. Abstr. in Zeit. fur Kryst. u. Min. vol. 3, 
pp. 626-629, taf. xv, figs. 6-11, 1879. 
