80 MINERALOGY AND LITHOLOGY. 
exactly the same, it may be assumed that this is the composition of our 
black mica. 
Biotite is hexagonal. In our quarries imperfect crystals are sometimes 
found, though pieces with an irregular outline are most abundant. It 
is quite common to find it united by the edges of its laminze with mus- 
covite or white mica; as if the muscovite, after reaching a certain size, 
had gone on increasing itself with the substance of biotite. Moreover, 
when the two species have thus grown together, it is found that their 
prismatic faces are parallel to one another. Although we do not often 
have the prismatic faces to study, still we have the means for finding 
their directions. When one takes a tolerably thin piece of mica, and 
strikes it quickly with a sharp point, lines of cleavage are developed 
about the hole made by the point; and thus a figure is produced called 
a strike figure (schlag figur). Now Reusch has shown * that this cleav- 
age in hexagonal mica is parallel to the sides of the prism, and that in 
orthorhombic micas it is parallel to the rhombic prism and the shorter 
of the lateral axes. Therefore the strike figure is exactly the same in the 
orthorhombic mica that it is in hexagonal, and in each case is composed 
of three cleavages, which cross one another at an angle of 60°. If, now, 
we strike with a pointed instrument upon a piece of this mica, which 
is composed of the two species, near the line upon which they are united, 
we shall produce these cleavage lines, which will run from one species 
into the other without interruption, and without any change of direction. 
This shows that there is some simple relationship between the positions 
of the crystalline planes of the two species; and, if we draw lines to 
represent the faces of the crystals parallel to these lines of cleavage, we 
shall obtain the correct positions of the crystals, and see their relationship 
to one another. Fig. 2 on Pl. 9 represents one of these pieces of mica 
with two of the strike figures,—one on the black mica and one on the 
white; andthe lines are seen to run without interruption from one into 
the other species. The faces of the crystals are drawn parallel to these 
lines, and the relationship of the two crystals is shown. 
Biotite is easily determined by the aid of the microscope. All sec- 
tions, save those parallel to the basal cleavage, are very strongly dichroic ; 
* Monatsbericht der Konigl. Akad. Wissensch. Zu Berlin, 1868, p. 428, and 1869, p. 84. 
