305 



homogeneous magma. The zircons of granites are generally less than the 

 1/1 00th of an inch in diameter. Tourmaline is an important constituent 

 of many granites. It occurs in crystals, crystalline grains and irregular 

 crystalline masses. The crystals belong to the hexagonal system and are 

 hemimorphic. Cross sections show in convergent light the figure of a 

 uniaxial mineral with negative double-refraction. Sometimes, however, the 

 black cross breaks up into hyperbolae as the stage is rotated in consequence 

 of optic anomalies. A zonal structure is often seen when cross sections 

 are examined in ordinary light. This is due to a variation in the colour of 

 concentric layers. The colour of tourmaline in thin section is very variable. 

 It may be yellow, brown, green or blue. Longitudinal sections are lath- 

 shaped and strongly dichroic. The maximum absorption always takes 

 place when the length of the section is at right angles to the short axis of 

 the polarizer. This furnishes a ready means of distinguishing tourmaline 

 from biotite. In tourmaline-bearing granites, such as those of Devon and 

 Cornwall, the amount of tourmaline increases towards the margin of the 

 principal masses and is often limited to the margins. It is, in part at any 

 rate, a secondary product, developed in connection with boracic and fluoric 

 acid exhalations. Where tourmaline occurs at the margins of granite 

 masses it is also found in the adjacent sedimentary rocks. Tourmaline 

 sometimes occurs in the form of acicular microlites which are often 

 aggregated into radiating bundles. This is well seen in Luxullianite. 



Topaz is so constantly present in greisen that it may be almost 

 regarded as an essential constituent. It is colourless and often without 

 form. Crystals of topaz have a short prismatic habit. The mineral is 

 colourless in thin sections. The double-refraction is about the same as 

 quartz. Cross sections show a positive bisectrix. The optic axial angle is 

 large, so that the axes are not included within the field of view in the 

 ordinary arrangement used with petrographical microscopes. There is a 

 perfect cleavage parallel to the basal plane. The mineral is probably 

 in all cases of secondary origin and developed in connection with fluoric 

 acid exhalations. It occurs not only in greisen but also in granite. 

 Granite shades into greisen by the disappearance of felspar ; the topaz to 

 a great extent taking its place. 



Fluorite is a constituent of certain granites and especially of the 

 variety known as trowlesworthite. 



Cordierite and its pseudomorph pin'tte are found in some granites. 

 Cordierite has been detected in the granite of Glencullen, Co. Dublin by 

 Mr. JOLY. tt> It shows the forms (100), (130), (110), (010) and (001). Finite 

 pseudomorphs after cordierite showing the forms (100), (110), (010) and 

 (001) occur in the granite of Breage and the Land's End in Cornwall and 

 in the quartz-felsite of Sydney Cove near Trewavas Head. Garnet is a 

 constituent of certain granites and occurs also occasionally in the trachytic 

 representatives of the acid group ; as, for example, in the quartz-felsite of 

 the Armboth dyke in the Lake District. Here it occurs in well-crystallized 



(1) Sci. Proc. Roy. Dublin Soc., 1885, p. 48. 



