826 SEC. 16. GEOLOGY AND MINING. 



5. Syenitic granite, Scale Force, Buttermere, x 30. 



6. Spherulitic felsite, Carrock Fell summit. (Polarized light), x 10. 



7 . Hypersthenite, White Crags, Carrock. (Polarized light), x 10. 

 1. Fine-grained band in hypersthenite. (Polarized light), x 10. 



3253g. Plates illustrative of Structures in Felspars, 

 laeucite, Obsidian, and Perlite, from drawings made partly in 

 pencil, partly in lamp-black, by Frank Rutley, F.G.S., H.M. 

 Geological Survey. Plates I. and II. illustrate a paper in the 

 Quart. Journ. of the Geological Society of London for Aug. 1875. 

 Plates III. and IV. illustrate a paper in the Royal Microscopical 

 Society's Transactions, March 1876. 



Frank Rutley, Geological Survey Office. 



PLATE; I. 



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 



curved 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 II. 

 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 



subordinate 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 

 alternately appear light and dark in different positions of the 

 Nicola when seen by polarized light, the overlap of the comple- 

 mentary 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. 



