i6 
The American (n'ologisf. 
Jaiiuary, 18'.W 
OOl 
Fig. i.— Manebacli. 
FtG. 5.— Carlsbad. 
FfG. C— IJavoiio. 
cleavage (oio) in one twin is parallel with that in the other, 
and unless the section be cut in a zone whose axis is either par- 
allel or perpendicular to the face oio, the different cleavages 
all form oblique angles with one another. If a section be in a 
zone whose axis is either parallel or perpendicular to the face 
GIG, the cleavages will stand at right angles. 
In the Baveno twin the plane of association is the clino- 
dome 02 1. Sections cutting such a twinned crystal present 
square or rhombic outlines, the cleavages being parallel to the 
sides. The line separating the twins runs diagonally, from 
corner to corner, as seen in figures 7 and 8. These sections 
are not uncommon, since in the case of the Baveno-twinned 
crystals they are also usually elongated parallel to the edge 
001:010. 
010 
010 
Baveno twins in tliin section. 
Fig. 7.— Perpendicular section. Fig. 8.— Obliciuc section. 
The basal faces and the brachypinacoids which form the 
surfaces of the prism are at right angles to each other. 
While these forms prevail in the monoclinic feldspars it is 
not uncommon that they unite, in the triclinic feldspars, with 
the albite and pericline types of twinning, which are rarely 
absent in the latter. 
Albite and Pericline Ttvimmig. — All the plagioclastic feld- 
spars are characterized by fine polysynthetic twinning, which 
