/S. L. Penfield — Mineralogical Notes. 117 



original error in the orientation. Another surprising and 

 unexpected peculiarity of this feldspar is the difficulty of pro- 

 ducing the cleavage parallel to (010). Among the specimens 

 that show the unusual pyramidal parting there is not one that 

 shows the (010) cleavage. Some are broken so that the frac- 

 ture runs nearly parallel to (010) but they present only the 

 curved surfaces of a conchoidal fracture. When the direction 

 of (010) is known, which can be determined by the extinction, 

 it is difficult to produce the cleavage by striking the blocks in 

 a favorable direction with the peen of a hammer, or by strik- 

 ing on a knife blade or chisel whose edge is placed on the crys- 

 tal in the direction of the desired cleavage. On specimens 

 which show the usual poly-synthetic twinning striae on the base 

 the (010) cleavage can always be readily obtained. This has 

 given rise to the query, is the good cleavage on plagioclase 

 parallel to (010) only apparent, and is it not possibly the result 

 of a separation along the plane of twinning % Certainly those 

 feldspars which show well developed poly-synthetic twinning 

 readily yield good cleavage surfaces, while those showing no 

 twinning, which the writer has been able to examine, usually 

 yield the (010) cleavage only with difficulty, so that it might 

 well be designated as rather poor and interrupted. 



In experimenting upon the cleavages of albite the following 

 peculiarities were also observed. The specimen examined 

 was a large crystal from Amelia, Ya., associated with quartz 

 and muscovite. It presented a combination of twinning accord- 

 ing to the Carlsbad and albite laws, similar to fig. 7, page 328 

 of the sixth edition of Dana's Mineralogy, and measured over 

 70 mm in the direction of the a and c axes. Two of the twin- 

 ning individuals were fully 6 mm broad, measured at right angleg 

 to the twinning plane ; they were clear and glassy, and free 

 from cracks and poly-synthetic twin lamellae. The crystal was 

 firmly united by the pinacoid (010) to the quartz and when 

 fragments were detached by striking with a hammer they were 

 always bounded by the basal cleavage and surfaces parallel to 

 the prism m(il0) and the pyramid o(IIl). These latter sur- 

 faces had the appearance of perfect cleavages, and often their 

 presence was revealed by cracks, running wholly or partly 

 across the detached fragments. The following measurements 

 were obtained on the reflecting goniometer, the reflections, 

 especially those from m, being sharp and well defined : 



Cy^tn, 001/sllO = 65° 10' mean of five independent measure- 

 ments, varying from 65° 7-65° 12'. 

 c^o, 001 a 111 = 51° 50' and 57° 33f. 



