84 MINERALOGY AND LITHOLOGY. 



of the two micas, as they appear between crossed Nicols without the 

 ocular, is also shown. These data give us the position of the crystals, 

 which are seen to be connected by their prismatic edges. 



In addition to that union of plates by a simple edge, there are also 

 found specimens in the mica quarries where plates of muscovite partially 

 or wholly surround a crystal of biotite; and here the same symmetry in 

 the arrangement of the prismatic planes is made evident by a determina- 

 tion of the positions of the planes of the muscovite. This was investi- 

 gated by Gustav Rose,* and Fig. 9 on PI. 3 is the one drawn by him 

 from a specimen from Alstead in our state. In this figure, a represents 

 the direction of the cleavage induced by a blow with a sharp point, and 

 b represents the direction of the plane of the optic axes, and the relation- 

 ship of the two figures is thereby easily identified. 



Every one who has seen the mica as it comes from these quarries must 

 have noticed how it is traversed at times by a straight crack, and that on 

 breaking it in two, this straight cleavage crack is filled with a multitude 

 of little fibres which are parallel to the direction of the line. At times 

 these natural divisions spoil a fine large sheet of mica, and at times a 

 piece of mica is divided by these divisions into a number of long strips. 

 Now Reusch and Bauer, who have studied the little cracks produced by 

 striking mica, have demonstrated that in muscovite a sharp blow with a 

 hard point develops a cleavage parallel to the sides of the prism and to 

 the shorter diagonal, as already shown ; but it was also found that by 

 pressure with a rounded point a little six-rayed star could be formed, the 

 arms of which were cleavages not parallel to the sides of the prism, but 

 at right angles to them. If, now, we take one of our sheets of mica with 

 a straight edge produced by natural division, and strike upon it with a 

 sharp point near the edge, we shall find that this edge is never parallel 

 to any of the little cleavage lines, but always stands at right angles to 

 one of them, and hence corresponds with one of the cleavages that can 

 be induced in the mica by pressure. This is illustrated in Fig. 10 on PI. 

 3. At a, a cleavage induced by a sharp blow is shown, which indicates 

 the direction of prismatic planes ; and at b, a cleavage induced by pres- 

 sure is shown, and the natural edges of this strip of mica stand at right 

 angles to a blow cleavage, and parallel to a pressure cleavage; and hence 



* Monatsbericht der Konigl. Akad. der Wissenschaften, Berlin, April, 1869, p. 339. 



