14 Mr. F. W. Rudler oh the 



succession of coloured bands, indicating its composite structure. 

 If cut parallel to the macropinacoid, or directly across the 

 lamellae, the striation comes out remarkably sharp, and some- 

 times as many as fifty parallel bands or lines may be detected in 

 a single crystal. In orthoclastic felspars such striation is not 

 seen. They present, between crossed Nicols, either a uniform 

 sheet of colour or a separation into two tints by a median 

 divisional line. The twins of orthoclase are generally con- 

 structed on what is termed the " Carlsbad type," fine twins of 

 this character being common in the granite of Carlsbad in 

 Bohemia. Here the two crystals are juxtaposed parallel to the 

 face known as the clino}nnacoid ; hence, when cut parallel to this 

 plane, no twin-structure is revealed ; but when cut parallel to 

 the orthopinacoid the line of junction is directly medial. The 

 orthoclase never shows the repeated twinning, or polysi/nthetic 

 structure, so marked in plagioclase. 



Some granites also contain a triclinic felspar termed micro- 

 dine, which exhibits in certain sections two sets of twin-strife at 

 right angles to each other, thus producing a cross-hatched or 

 grating or chequered structure. 



It was shown some years ago by Prof. Reuscli, of Tiibingen, 

 that pressure applied to calc-spar may induce a twin structure ; 

 and hence it has been suggested that in certain rocks the 

 structure of the felspar may in like manner have been develoved 

 by mechanical means. This is especially probable in felspars in 

 which the twin-bands end with abruptness. 



Although it is difficult to distinguish the different species of 

 plagioclase by microscopic means, yet some approximation may 

 be made by the "method of extinctions." The Nicols are 

 crossed, so that the field becomes dark ; the section is then 

 introduced, and in consequence of depolarisation more or less 

 light is admitted. One of the spider-lines of the eye-piece is so 

 adjusted that it runs parallel to a selected edge of the crystal 

 under examination. The stage bearing the crystal is then 

 rotated, until extinction occurs ; that is to say, until the field 

 again becomes dark. The amount of rotation is then measured 

 on the graduated edge of the stage, and this gives the angle of 

 extinction in relation to the edge in question. Professor Max 

 Schuster, of Vienna, has drawn up a table of extinctions for the 

 diagnosis of the felspars. 



Quartz, as a constituent of granite, occurs in the form of 

 crystalline grains rather than as distinct crystals. Unlike the 

 felspar, which is generally turbid, the quartz is pellucid ; but it 

 usually contains pores or cavities, disposed in linear series, 

 looking like lines of dust. These cavities frequently enclose 

 liquid, with a movable bubble, like that in a spirit-level. In 

 polarised light the quartz displays vivid colours, which generally 



