300 



tion. Sections parallel to the macro- and brachy-pinacoids show only one 

 set of lamellae and these planes correspond, therefore, approximately to 

 the faces of composition. The want of persistence in individual lamellne is 

 strongly marked in these sections. Irregular patches of albite or oligoclase 

 are almost always found intergrown with microcline so that the M faces and 

 the vertical axes approximately coincide. These are best observed in 

 cleavage flakes or sections parallel to P where the even lamellation of the 

 albite is in strong contrast to the cross-hatching of the microcline. The 

 chemical composition, refraction, double -refraction and specific gravity are 

 approximately the same for microcline as for orthoclase. According to 

 M. LEVY the two minerals are identical, the symmetry and optical characters 

 of orthoclase being accounted for on the assumption that it is, so to speak, 

 an ultra-microscopic microcline ; that is a microcline in which the 

 polysynthetic structure is so minute as to be incapable of resolution by our 

 methods of examination. One and the same individual may show the char- 

 acters of orthoclase in certain parts and those of microcline in other parts. 



The alteration of microcline is similar to that of orthoclase. Under 

 the influence of dynamic metamorphism it passes over into the mosaic-like 

 aggregates already referred to. 



Soda and Soda-lime Felspars. Normal plagioclase felspars are present 

 in almost all granites and occur not unfrequently in the trachytic rocks of 

 similar composition. They belong for the most part to the oligoclase-andesine 

 group. Albite is present in certain granites but appears to occur rather in 

 drusy cavities, or intergrown with orthoclase, than as an independent rock- 

 forming constituent. Labradorite has been found in the dark patches so < 

 common in many granitic rocks. In general character and mode of 

 association the soda-lime felspars of the present group of rocks are similar to 

 those of the intermediate group and require therefore no special description. 



Quartz. In the granitic rocks this mineral usually occurs in the form 

 of grains or granular aggregates, less frequently in the form of ophitic 

 plates with uniform extinction over considerable areas. Idiomorphic quartz 

 is comparatively rare in the granitic rocks. Some rocks which have been 

 termed granites (e.g., rocks of Mull) exhibit the micro-pegmatitic structure 

 in the greatest perfection. These may with propriety be termed granophyres 

 notwithstanding the fact that porphyritic constituents are sometimes absent. 

 In the trachytic rocks quartz occurs as one of the porphyritic constituents. 

 It is then sometimes idiomorphic. The usual form is that of a double 

 pyramid ; the prismatic faces being absent or developed only to a very 

 limited extent. As a general rule the angles are more or less rounded and 

 sometimes the mineral occurs in the form of grains without any decided 

 indications of crystalline form. Liquid inclusions are very common in the 

 quartz of granites. They may lie in planes or may be irregularly distributed 

 through the mineral. Details as to these inclusions have already been 

 given and need not therefore be repeated. Liquid inclusions are generally 

 absent in the trachytic rocks. /Inclusions of the ground-mass are common 

 and these not unfrequently take the form of negative crystals. The 

 boundaries of the crystals and grains of quartz in the trachytic rocks are often 



