352 W.F. Hillebrand—Léillingite and other Minerals. 
shows a continuation in some degree of the prismatic striation 
parallel to the same combination edge. 
The first step toward the complex twin structure is the forma- 
tion of a simple twin, or rather trilling, by interpenetration of 
three single crystals having the basal section in common, an 
face of the prism as the composition face (fig. 3). A basal sec- 
tion shows a six-rayed star, with angles of very nearly 60°, by 
microscopic measurement, between the axes of the rays. In 
the microscopic twins one individual frequently predominates 
greatly in size over the other two, these appearing often as 
thin leaves, projecting but a short way out of the larger crystal. 
These trillings are finally found again interpenetrating, not ac- 
cording to any recognizable law, but seemingly in every direc- 
tion, and in indefinite numbers, forming thus the complex 
aggregates first spoken of. All these stages of change in form 
may be observed with. great ease under the microscope, the 
very smallest crystals alone showing crystallographic faces ‘well 
defined. s the crystals, single or twin, increase in size, the 
faces gradually grow more and more indistinct, and finally dis- 
appear entirely in consequence of increasing striation. 
Notwithstanding repeated attempts, the basal cleavage men- 
tioned in text-books as characteristic of léllingite could rarely 
be produced, and never a cleavage in any other direction, eX- 
cept in the case of the trillings. Here an individual frequently 
broke off at the line of union of the three, that is, in a plane 
parallel to the brachy pinacoid. . : 
side from the forms distinctly recognizable as Jdllingite 
are, however, others belonging, apparently, to two different 
minerals. The first of these became visible on dissolving the 
gangue, when there came to the surface of the aci and the 
water used for washing out the latter a great number of minute 
but brilliant metallic particles which resolved themselves under 
the loupe, and still better under the microscope, into thin leaves 
or blades, of which fig. 4 represents one of the more perfect 
examples. Its forms appear to consist of two pinacoids of the 
rhombic system, one very broad, the other very narrow, and a 
lengthened prism with an angle of very nearly, if not quite, 
90°. Repeated measurements gave values fluctuating between 
