THE NATURAL HISTORY SURVEY. 4! 



(Lehrbuch, d. Min. 1894, p. 471,) calls the two minerals hetero- 

 morphous. Hintze (Handbuch der Min:, 1897, Vol. II, p. 1,335) 

 states that a separation of potassium feldspar into monoclinic 

 and triclinic is not possible. Dana, (Text Book, Min. 1899, P- 

 374), recalls the statement made by Mallard and Levy more 

 than a quarter of a century ago that orthoclase and microcline 

 are essentially identical. 



Plagioclase. 



The plagioclase feldspars are represented by several varie- 

 eties, chiefly by oligoclase and labradorite. They have the 

 same planes as orthoclase and angles which differ but slightly 

 from that species. They are, however, triclinic. The base 

 (ooi) forms an acute angle with the left brachypinacoid (oTo), 

 and an obtuse angle with the right brachypinacoid (oio). The 

 pyramid and dome planes sometimes are developed on one 

 side and not on the other, thus showing the unsymmetrical 

 character of the crystals. Carlsbad, Baveno and Manebach 

 Laws of crystallization obtain with this species, but the most 

 common law of crystallization is that known as the albite law, 

 which shows on a section parallel to the base multitudes of 

 thin lamellae parallel to the brachydiagonal. When we have 

 twinning according to the Pericline law, that is, parallel to the 

 macrodiagonal, the polysynthetic striations aie evident on the 

 brachypinacoid and on the basal plane as striations at right an- 

 gles to those of the albite law, and this causes grating structure 

 as seen in microcline. The cleavage parallel to the base and 

 brachypinacoid is perfect; parallel to the lefthand prism, im- 

 perfect. Because of the twinning parallel to the edge made 

 by the base and brachypinacoid, the basal plane often appears 

 striated, and this is so characteristic that when seen causes us 

 to pronounce the species a plagioclase. Hardness, 6, specific 

 gravity, 2.6 to 2.7. Colorless when pure, colored by enclosures 

 grey, green and red. These are soda lime feldspars. Some of 

 them show as high as 25% of calcium and have been classed 

 as oligoclase, while others show nearly 75%, and are classed 

 as labradorite. (See Szabo "Trennung's Methoden"-Pest, 

 '76). The specimen determined as oligoclase was insoluble in 

 acid, while that pronounced labradorite was slightly soluble. 

 The oligoclase fragments melt more easily than do orthoclase. 

 The containing rock is a diorite porphyry. Labradorite occur- 

 ring in this region exhibits no play of colors. The blue and 



