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quartz jintl feldspar, with occasionally a little biotite. The microscopical habit of the.se 

 porphyritical crystals is so constant in all the thin sections of this group as to permit 

 of a single detailed description, the different modifications of the groundmass only 

 requiring special notice. The feldspar present is sauidine, with which plagioclase is 

 associated to a greater or less extent. The latter is in some cases entirely wanting, 

 but in others is almost as abundant as the sanidine. Sometimes both occur in very 

 small quantities in the thin sections and hardly ever outnumber the quartz. Sani 

 dine occurs in well developed crystals and also in angular fragments. Sections of 

 the former are mostly rectangular, with the corners rounded; others show more than 

 four sides and indicate that their crystal form is made up of OP, ooPoc, ooP, 2P* 

 Zonal structure is rarely observed. Many of the individuals are in Carlsbad twins. 

 The cleavage is frequently very perfect, though often entirely wanting, but there 

 are always concoidal fractures, and the resemblance to quart/, is often very striking, 

 requiring an optical test to distinguish between them. It is characterized by a much 

 lower double refraction, which in these extremely thin sections causes it to remain 

 generally dark or but faintly lighted between crossed nicols. Quite a number of 

 sections happen to be nearly at right angles to the optical bisectrix and exhibit very 

 small angles between the optic axes, the interference figure being almost a cross and 

 showing the bisectrix negative. There are several of these in thin section 112. A 

 fortunate section parallel to the clinopinacoid occurs in thin section 142 and is at 

 right angles to the optical normal, which is found to be positive, the interference figure 

 being hyperbolas that unite in the center of the field. The inclination of the plane of 

 the optic axes is about + 7 to the basal cleavage, and the angles of the sides of the 

 feldspar section correspond to those cut from P, ocP, and 2P oc. Besides the basal 

 cleavage, which in this section is very perfect, is a second less regular cleavage 

 parallel to the trace of the orthopinacoid. The plane of the optic axes in these sani- 

 diues is sometimes in the plane- of symmetry, sometimes at right angles to it. The 

 substance of the sanidine is very pure and free from inclusions of foreign matter. 

 Numerous minute gas cavities, however, occur irregularly scattered, some of which have 

 their sides wet with fluid, but the gas has always the greater volume. A notable 

 exception to this freedom from inclusions occurs in thin section 141, Fig. 2, PI. v, 

 where two sharply outlined crystals of sauidine grown together with different orien- 

 tation about a fragment of plagioclase are filled with quartz in orderly arranged 

 forms, with constant crystallographic orientation throughout certain portions of the 

 feldspar crystals, which is shown by the extinction of light and the parallel position of 

 numerous small dihexahedral glass inclusions with gas bubbles, found only in the 

 quartz, whilst irregularly shaped gas cavities occur in the feldspar substance. This 

 is a most interesting fact from its relation to the subject of fluid and glass inclusions 

 in volcanic rocks, for it would appear from this instance that quartz and feldspar crys- 

 tallizing out at the same time and under the same conditions have inclosed, the one 



