MINERALS—CHIASTOLITE OR MACLE. 305 
they stand out above its surface, and are thus distinctly presented to view, forming beautiful 
cabinet specimens. The crystals are mostly exposed on the ends, and they appear rectangular, 
The specimens obtained at the Chowchillas River, and in the conglomerate, are all of the 
variety of andalusite called chiastolite, or macle, formerly considered a distinct species. This 
variety presents characters which give it a peculiar interest. When the prisms are broken, or 
ground down at right angles to the main axis, remarkable geometrical figures are exposed. 
These consist of black lines, or masses, regularly and symetrically disposed around the centre 
of the crystal, and generally occupying the diagonals of the prism. Various designs are thus 
produced, and they generally take tht form of a cross. These peculiarities will be better under- 
stood from the following figures, taken from specimens obtained at the locality. 
5 
. 6 
CRYSTALS OF ANDALUSITE OR MACLE. 
These peculiar geometrical figures, so often forming a perfect cross, rendered this mineral in 
ancient times an object of search for ornamental purposes and for devotional uses. Opposite 
ends of the same crystal often do not show the same arrangement of the black portions, as 
shown in figure 7. The central black prism, or nucleus, is often pyramidal in form; and at 
one end may be nearly as broad as the whole crystal, but at the other appears as a mere point 
inthecentre. This is well shown in one of the specimens taken from the bed of the river, 
which has been cut and polished by the lapidary in four places. Although the outer surface of 
the crystal is dark-yellowish drab in color, the interior is quite light, and has a delicate shade 
of yellow and gray. A brilliant internal reflection. from a multitude of points in the mass of 
the mineral is also seen, and when it is held in certain positions it much resembles some speci- 
mens of feldspar. In this light-colored ground the dark, black lines of a cross, or central 
rectangle, show with distinctness, forming. specimens of great beauty. At one end of the 
erystal this rectangle is quite small, and forms almost a mere point in the centre of the crystal ; 
but each succeeding section, towards the other end, reveals its enlargement, until it finally 
includes the great body of the prism within its sides. : 
We must look for an explanation of the formation of these peculiar characters to the arrangement 
of impurities in the body of the crystal by the forces of crystallization. "This explanation was ad- 
vanced by Beudant,! and is supported by the specimens under consideration. Similar phenomena 
are exhibited in crystals of different salts formed in a fluid containing light impurities. "These 
become inclosed in the mass of the crystal without materially modifying its form, and are 
found distributed in symetrical figures, their form and direction depending on the system of 
crystallization. The impurities in the crystals of andalusite consist of portions of the sur- 
rounding mica-schist, and a specimen is figured by Dufrénoy, showing the passage of the central 
black prism into the surrounding rock. This portion has the schistose structure of the rock, 
and gradually tapers to a point, while the white portions of the crystal increase in thickness as 
! Traité de Mineralogie, Beudant. 
39 F 
