604 The American Naturalist. [July, 
is more common than the unit pyramid. The etched figures on the 
base indicate hexagonal, or possibly rhombohedral symmetry. 
In agreement with the observations of de Bournon, Bauer and others, 
Judd’ considers the structural planes of corundum to be not true cleay- 
ages, but simply planes of parting. Those parallel to OR (0001) and 
æ P2 (1120) are thought to be normal solution planes, while the set 
parallel to R (1011) are explained as gliding planes which, after the 
gliding under pressure, have become secondary solution planes. Ob- 
servations on the shape and crystallographic position of the solution 
cavities are recorded. These are either empty or filled with the pro- 
ducts of alteration. 
epre gives a full list of the literature of enargite, as well as of its 
es of occurrence. He cites for the first time the ten new planes, 
(610), (520), (540), (230), (108), (207), (709), (301), (601) and (054). 
Of these (610) and (601) are noted as doubtful. The mineral clarite 
(Sandberger, N. J. B., 1874, p. 960 and 1875, p. 382) is considered to 
be identical with enargite. 
The relationships of the members of the humite series are reviewed 
by Lewis,’ who brings the orthorhombic pyroxenes, as well as olivine, 
into the discussion. Besides the well-known progression in the axial 
ratios, the remarkable fact is noted that the twin face in all these min- 
erals is parallel to the b axis, and in all except clinohumite it makes 
an angle of about 30° with the base (001). In clinohumite it has a 
position about at right angles to that in the other minerals. Another 
common feature is the great predominance of forms lying in the zones 
010, 110 and 210. The minerals are similar in density, hardness and 
fusibility. The moleculare volumes are :— 
Enstatite, $2.8.== $x 21.9 
Olivine, fo oe Oe Oe 
Chondrodite, 105.6 = 5x 21.1 
Humite, lo = 7 x 21.4 
Clinohumite, 182.1 — 9x 202 
(Note—The molecular volume of clinohumite is probably too low, 
as given here. From the first three minerals one might compute the 
volumes of MgO (11.2), SiO, (21.6), and Mg,O (OH, F), (28.8). This 
would give for humite the molecular yolume 149.6, and for clinohu- 
mite, 193.6.) 
1 Min. Mag., Vol. XI, Sept., 1895, pp. 49-55. 
8 Min. Mag., Vol. XI, Sept., 1895, pp. 69-79. 
* Min. Mag., XI, Oct., 1896, pp. 137-140. 
