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Transactions. — Geology. 



composition plane is a continuation of that of the plagioclase. 

 Since these two planes are coincident, the plane of twinning on 

 the Carlsbad type in the plagioclase must be parallel to the 

 clinopinacoid in the sanidine. But the twinning-plane of the 

 albite lamellae is parallel to the twinning-plane of the Carls- 

 bad twins, and, as, according to the albite law, the lamellae are 

 twinned parallel to the brachypinacoid, it is evident that the 

 composition plane of the Carlsbad twins is also the brachy- 

 pinacoid. Hence in these crystals the clinopinacoid of the 

 sanidine corresponds with the brachypinacoid of the plagio- 

 clase. The Carlsbad twins in the plagioclase show the same 

 irregular intergrowth that forms a noticeable feature in the 

 sanidine. 



Albite twinning is exceedingly common, the lamellae vary- 

 ing greatly in width, but on the whole they are narrow, the 

 width being constant throughout their length. In a few cases 

 the lamellae are curved or broken, and possess undulose extinc- 

 tion, thus showing that the rock was subjected to consider- 

 able pressure or tension previous to its extrusion. 



If a section of plagioclase is cut at right-angles to the 

 brachypinacoid the extinction of the adjacent lamellae of the 

 twins would make equal angles on each side of the cross-wires. 

 Although in the sections none of the plagioclase crystals are 

 cut precisely in this direction, some of the surfaces coincide 

 approximately with such a plane. 



The following have been measured : — 



These results, although rather high, tend to show that the 

 species of plagioclase is andesine, a conclusion that will sub- 

 sequently be shown identical with that derived by chemical 

 analysis. It was found impossible to detach cleavage-llakes, 

 so that the extinctions on the base or brachypinacoid could 

 not be determined. 



Twinning after the pericline law (twinning-plane the 

 " rhombic section ") is not nearly so frequent nor so well 

 developed, the lamellae being as a rule of variable length, fre- 

 quently not traversing the whole breadth of the crystal, and 

 leaving untwinned feldspar between the adjacent twins. 



Plate XL VIII., fig. x., shows a crystal where this type of 

 twinning is seen in combination with twinning after the albite 

 law. The drawing is a faithful representation of the crystal as 



