RoaErs. | Minerals of the Galena-Joplin District. 489 
TWINNING. 
The calcites of the district are twinned according to the 
four laws known for calcite, the twinning planes and com- 
position faces being, (1) O,OR; (2) S-,—¢R; (8) p-,+R; 
(4) g-,—2R. Of these, the first two are much more common 
than the other two. 
Twins, according to the first law, were observed on crystals 
of types 3 and 4 by Farrington, and on crystals of types 2, 3, 
4, 22 and 23 by the writer. Figure 61 gives an idea of the 
average development of the twins of type 22, which are fairly 
abundant at the Gracie Clark mines, near Galena. Twins of 
type 23, represented by figure 62, are from the B. C. mine, 
west of Joplin. 
Twins in which the twinning plane is S.,—4S are not rare. 
Farrington mentions crystals with the forms K:, ¢., S-, and 
2. which he does not include in any of his types, as exempli- 
fying this mode of twinning. Figure 64, copied from Far- 
rington, represents one of these crystals. Twin crystals of 
type 7 with the twinning plane S- are of frequent occurrence. 
A single example of this twinning law was observed on type 
13, from the Homestake mine, at Granby, and on type 4 from 
Aurora, the latter in the collection of Mr. G. W. Kane. At 
the Elizabeth mine a large number of cleavage fragments 
twinned on S- were found. (Fig. 65.) Striations on the 
cleavage surfaces of calcite parallel to the shorter diagonal 
of the rhomb are commonly observed. These striations may 
also be easily produced artificially by the blow of a hammer 
or other tool. Opaque white cleavage pieces of calcite are 
often found on the dump-piles. Such, the miners say, are 
always found in the vicinity where blasting has occurred. 
From the examination of specimens, it seems probable that 
their opacity is due to minute cracks produced by twinning 
on S- and by cleavage. 
The third method of twinning (twinning plane p-,+R) was 
observed on a single crystal from Joplin, in the Bement col- 
lection at the American Museum of Natural History, New 
York city. 
The forms present are b,~R,andS8-,—3R. The individual 
