MICROSCOPIC STRUCTURE OF FELSPARS. 487 



EXPLANATION OF PLATES. 



Plate XXIII. 



Fig. 1. Orthoclase, Arendal, showing crystals arranged in two directions, other 

 than those of striation. X 50 (polarized light). 



2. Felspar crystal in perlite, Schemnitz, Hungary, showing partial cross- 



hatching. X 55 (polar.). 



3. Crystal in trachyte, Berkum, near Bonn, Rhine, showing internal curyed 



divisional markings, with lines crossing at right angles in the lateral 

 areas. X 115 (polar.). 



4. Partially formed, or partially disintegrated crystal in obsidian, Mexico. 



X 175 (dark-ground illumination). 



5. Patches of cross-hatched felspar in oligoclase, Twedestrand, Norway. 



X 115 (polar.). 



6. One of the above patches, showing partial cross-hatching. X 350 



(polar.). 



Plate XXIV. 



Fig. 1. Minute crystal in obsidian, Mexico. X 240. 



2. Diagrammatic exaggeration of the above. 



3. Crystal from trachyte, Berkum, Rhine. X 90 (polar.). 



4. Crystal from basalt, Cleveland, showing rectangular banding and subor- 



dinate oblique striation, the latter possibly indicating planes connected 

 with twinning on the Baveno type. X 45 (polar.). 



5. Crystal in obsidian, Mexico. X over 100. 



6. Crystals of plagioclase in gabbro, Volpersdorf, Silesia, showing bands 



at right angles. X 90 (polar.). 



7. Crystal in basalt, Cleveland, Yorkshire. X 45 (polar.). 



8. Fragment of plagioclase in basalt, Cleveland, showing alternately dark 



and light bands fringing an obliquely cut twin lamella, as in fig. 10. 

 X 45 (polar.). 



9. Crystals on edge of section of basalt, Cleveland, showing cross-hatched 



striation and a rectangular cleavage at x on the outer edge of the 

 section. X 45 (polar.). 



10. Diagrammatic vertical section through a section of plagioclase cut 



obliquely to the twinning -planes : the spaces marked w would alter- 

 nately appear light and dark in different positions of the Nicols when 

 seen by polarized light, the overlap of the complementary colours 

 giving rise to white light. 



11. Diagrammatic section similar to fig. 10, but cut more obliquely to 



the twinning-planes, so that, instead of coloured bands separated by 

 light or dark ones, an unbroken surface of white light might result, 

 although the section might be that of a felspar many times twinned. 



Discussion - . 



Mr. Koch thought that the structure of the crystals depended 

 greatly upon the manuer in which the rock was formed and the 

 treatment which it had subsequently undergone. Exposure to high 

 temperatures will cause the angles of crystals to alter greatly; and 

 he had thought that felspars might be used as pyrometers for this 

 reason, but on trying the experiment found that various changes 

 in the angles were produced by exposure to the same temperature. 

 He noticed that crystals formed in the humid way, heated and sud- 

 denly cooled, will present exceptional forms. He could not under- 

 stand the curved lines shown in the crystal from Mexican obsidian. 



