PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 135 



by raeans of the measurement of the maximum angle between the twin- 

 ning plane and the elasticity planes in thin sections cut in the zone with 

 its axis perpendicular to the twinning is followed. 



I wish also by means of an analysis to draw a,ttention to the circum- 

 stance, that the absence of a twinning plane parallel to the bracbypin- 

 acoid is by no means rare, and this is a circumstance bearing upon the 

 same point and demanding recognition in lithological woik. 



At the St. Paul Island, Labrador, from whence the beautiful striated 

 labradorite specimens are obtaiued, there are also found large pieces of 

 feldspar with beautiful broad basal cleavage planes, which show no trace 

 of striation, and basal sections of which in polarized light under the mi- 

 croscope show no trace of twinned structure. 



A piece of this feldspar in the cabinet of Professor Brush, at oSTew 

 Haven, is of a bluish-gray color, its basal cleavage is much more perfect 

 than is usually the case with labradorite, and its cleavage parallel to the 

 clinopinacoid is very much poorer than usual. It only shows in the 

 latter direction a conchoidal vitreous fracture, with mere traces of cleav- 

 age. Its dark color is imparted to ^t by a multitude of minute black 

 needles, the larger of which are iidaid parallel to the vertical axis of the 

 crystal. The intersection of an elasticity plane and the base is incliued 

 50 to the edge between the basal and brachypinacoidal cleavages, while 

 the intersection of the plane of the optic axes and the brachypinacoid is 

 inclined 17^ to the same edge, and both inclinations are in the directions 

 characteristic of labradorite. 



An analysis of this feldspar by Mr. F. W. Taylor, of the National 

 Museum, yielded him the following result: 



9.3 



1. 



The angles and analysis therefore show that this is a typical labra- 

 dorite, and as it forms at the St. Paul Island a component of a coarse- 

 grained rock, it may be considered as a type of a triclinic feldspar, -which 

 could not be determined in a thin section b^' means of any method based 

 npon the existence of twinning i)lanes. 



I would also call attention to some other cases. 



The diorite from Dixville Notch in the White Mountains, described on 

 page 96 of my report on the Lithology of New Hampshire,* contains 

 * Geology of New Hamxjsbire, by Prof. C. H. Hitchcock, vol. III. 



