OPTICAL CHARACTERS OF MINERALS. 253 



it usually is blue on one side of the twin plane and yellow on the 

 other, but the colour varies with thickness of the slice, &c. 



Oligoclase exhibits many alternations of blue and yellow in thin 

 bands. This condition of repeated twinning is common in triclinic 

 felspar. 



Labradorite only assumes a darker tinge when the polariser is 

 rotated, and shows banded structure due to twinning. 



Anorthite is distinguished from labradorite by showing brighter 

 colours than the other felspars. 



Nepheline. In thin slices its colours are very pale, and vary, 

 between crossed nicols, from dark milky-blue to brownish-yellow. 



Leucite shows no colour, though when the slices are sufficiently 

 thick the mineral has a bluish-white tint, paler than nepheline. 



Black Mica gives green, yellow, and brown tints when the sec- 

 tion is hexagonal ; when cut across the lamellae, and the plates are 

 very thin, there is a carmine tint. 



White Mica, in which the divergence of the optic axes is greater, 

 shows yellow and red colours, but the colours are clear. 



Amphibole. In this the colours may be either pale green or blue, 

 or yellowish, or dark brown between crossed nicols. They are brighter 

 than those of pyroxene. The Actinolite variety gives emerald green ; 

 glaucophane, clear blue; hypersthene is intermediate between 

 pyroxene and amphibole ; epidote is citron yellow to brown. 



Pyroxene. The colours of augite are not so clear as those of 

 epidote, nor so bright as those of olivine. They are often yellow and 

 red, sometimes green-brown and rose, varying with the rock. 



Olivine in very thin slices is colourless, but in thicker sections its 

 colours are brilliant red and green. Apatite varies from pure white 

 to bluish white or yellowish. 



Titanite between crossed nicols gives deep-yellow and brown 

 colours, but less bright than those of augite and hornblende. 



Tourmaline is brown or dark green, rarely blue or rose colour. 

 It may be compared with hornblende, biotite, and epidote. 



Frequently it is necessary to measure the angles of crystals to 

 identify some minerals with certainty ; this can be done by means of 

 graduated circles and a rotating stage, such as are found in Rosen- 

 bush's petrological microscope. We have also used crossed spider 

 lines in the eye-piece, one of which is capable of being rotated, as an 

 instrument for measuring crystal angles under the microscope, with 

 the advantage that the object is not moved. 



Texture of Igneous Rocks. The structures of igneous rocks 

 which are most easily recognised, admit of being classed according to 

 the conditions of solidification into three types : first, crystalline ; 

 secondly, semi-crystalline ; and thirdly, uncrystalline. These divisions 

 are adopted by Professor Zirkel in his "Microscopical Petrography." 



(i.) Crystalline Rocks. Granite is the best type of a crystalline 

 rock, in which the texture is macroscopic, or such that the crystals 

 may be distinguished by the naked eye ; but the same type of crystal- 

 line structure may exist when the texture is cryptocrystalline, or such 



