1885.] Mineralogy and Petrography. -597 
position of these planes was determined for gypsum, stibnite, bis- 
muthinite, orpiment and cyanite, and found to agree as regards 
their direction with what had earlier been observed in the case of 
mica, rocksalt, calcite and galena. Later, the application of Reusch’s 
method of forming the so-called fracture figures (‘Schlagfiguren’’) 
showed that the same was also true of erythrite, vivianite, hydrar- 
gillite, brucite, potassium ferrocyanide, uranium mica and even 
apophyllite and topaz. In the case of the two latter minerals, it 
was necessary to use a diamond-pointed instrument to make the 
impression.” Most interesting, however, are these “gleitflachen” 
in calcite, where they stand in the closest relation to the twinning 
planes. As “ structure planes” Migge designates all those along 
effected. Of ea in calcite R is the true cleavage plane (“ Spal- 
tungsflāche ”); —1%4R the slipping plane (“ Gleitfläche ”), while 
parallel to %P2 and oR parting readily takes place under press- 
ure in the proper direction (“ Reissflächen ”). If an artificial 
sliding of a portion of a calcite rhombohedron into twinning 
position parallel to — YR be effected, the position of the R-faces 
remains unchanged ; the — 14 R-faces, except the one in which the 
sliding took place, assume the position of oo P2 planes, while these 
in turn come to occupy the place of —%R. A oR-face takes 
the position of —2R, parallel to which a parting has also been 
observed by Haidinger and Tschermak. Thus all the structure 
planes retain the same positions, although they exchange them 
among themselves. It is significant that all these planes, except 
œP2, which is a plane of symmetry, have been observed as twin- 
ning planes for calcite The great ease with which by slight 
pressure the molecules of calcite may be pushed into twinning 
position parallel — 4R suggests a similar origin for the lamellar 
twinning of many silicates, especially as this is often observed to 
be most developed w where the pressure has been greatest. Van 
Werveke has called attention to this in the case of feldspar and 
diallage ;* Bauer, in the case of cyanite.® Analogous examples 
seem to be Malacolite and salite ( | oP), epidote and mica. [The 
appear to so ai to this same category, and may ‘testy have 
been produced by pressure. Hornblende rarely exhibits the same 
structure parallel tooP.]’ Migge also describes twinning lamellz 
in imbedded masses and crystals of hematite and corundum. They 
1 age er für Min., etc., 1883, II, p. 13. 
7 Ih, . 50. 
? eles poser fiir Min., etc., 1883, Ipp. 32. 
4 Ib., 1883, 1, p 
§ Zeitschrift der deteochen geol. Gesell., 1878, xxx, p. 320. 
€ Amer. Jour, of Science, June, 188. 
VOL. XIX.—NO. VI. 39 
