I, GEOLOGY. 699 



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 

 Nicols 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. 



PLAIE III. 



Figs, 1, 4, 5, 6, 7. Structures seen in a thin section of spherulitic obsi- 

 dian from the lava stream of JRocche Ross'e, in the Isle of Lipari. 

 8. Portion of one of the spherulitic bands in the above rock, x 22. 



12. Magnetic crystals partially converted into peroxide of iron, occur- 



ring in a crystal of leucite from Vesuvius, x 760. 

 2, 3, 9, 10, 11, 13, 14. Structures seen in a crystal of leucite, from 

 Vesuvius. 



PLATE IV. 



Structure in perlite, from Buschbad, near Meissen, showing the spheroidal 

 structures lying between divisional straight lines F. F. F. F. 



3253. Map showing the work of the Geological Survey of 



Scotland, on the scale of one inch to the statute mile. All the 

 published sheets of the map are given here, and a few of which the 

 survey is completed, but which are not yet engraved, are inserted 

 in MS. At the foot of the map an example of the horizontal 

 sections is given which are run across the country on the scale of 

 six inches to the statute mile. 



Geological Survey of Scotland, Prof. Geikie, F.R.S., Edin- 

 burgh, Director. 



3254. Geological Map of the Ayrshire, coal-field and adjoining 

 districts, on the scale of six inches to the statute mile. This map 

 has been selected as an illustration of the detailed work of the 

 Geological Survey of Scotland. The whole county is surveyed on 

 this scale, though only the mineral districts are published on these 

 maps, the general map of the country (see No. A.) being on the 

 scale of one inch to a mile. At the foot of the map a MS. sheet is 

 inserted to show the stages of progress in the field work of the 

 survey. Two specimens are likewise given of the detailed vertica 



