810 The American Naturalist. [October, 
Crystals of lead nitrate and of barium nitrate also develop transitory 
faces when rounded, returning to the original form of cubo-octahedron 
on continued growth. Miers has observed the formation of the face 
(221) by the extremely slow solution of the potassium alum crystals. 
Hence, it seems that these “ transitory planes” may be formed either 
by corrosion or by growth of a rounded crystal. 
Albite from Lakous, Island of Crete.— Viola, by his paper on 
the new occurrence of albite at Lakous, adds another to the list of 
carefully investigated pure chemical substances. An analysis by 
Mattirolo, given at the end of the article, shows close agreement with 
the theoretical values for Na Al Si,O, as may be seen from the follow- 
ing: 
Found. Theoretical. 
SiO, 68.35 68.70 
Al,O, 19.78 19.47 
Na,O 11.71 11.83 
K,O 16 
Ign 15 
100.39 100.00 
Measurements on twelve crystals, varying from 73 to 20 mm. in di- 
ameter, agree very well in giving as crystallographic constants: = = 
94° 14’ 30”, 8 — 116° 31’ 45”, y = 88° 5’ 1”, and 4: b : e = .635: 1 :.557. 
The extinction angle measured against the trace 001 in a section cut 
parallel to 010 is 21° 30’, in the section 001 it is 3°30’. The optical 
angle is approximately -+ 80°. Inclusions of a member of the chlorite 
group are found in a number of the crystals, and some small scales of 
hematite in others. 
Forsterite from Monte Somma.—The specimens seem to be of 
unusual chemical purity, hence the data given by Arzruni* on the 
the basis of investigations by himself, Jolles and Thaddéeff are doubt- 
less near the true values for pure Mg, SiO,. The axial ratio is found to 
be a: b : c =.46663: 1: .58677. Cleavage parallel to 010, distinct. 
In addition to the previously observed method of twinning, the plane 
031 is reported as a twinning plane. 
3Tscherm. Mitth., XV, pp. 135-158, 1895. 
` 4Zeitschr. f. Kryst., XXV, pp. 471-476. 
