PETEOGEAPHY— EHYOLITES AND TEACHYTES. 487 



differing from that of granite and gneiss, &c, is the absence of any fluid 

 inclusions,* so usual in the latter rocks. Instead of these are particles 

 of volcanic glass, called glass inclusions, which are never found in granitic 

 rocks. The crystals of quartz are generally of sharpl}' defined outline 

 and often twinned, which latter occurrence is most clearly seen in polar- 

 ized light, as shown in [127], Plate I, Fig. 1. 



Sanidin, the chiet constituent of trachytic rocks, occurs in transparent 

 crystals, full of fissures, cracks and cleavage lines. The crystals in rhyo- 

 lite, however, are not so large as those usually in sanidin-trachyte. The 

 form and the crystals as seen in the thin section varies greatly, from large, 

 rectangular sections to long, columnar crystals, the latter being often very 

 small. The sections are also six and eight-sided Carlsbad twin crystals 

 are of frequent occurrence, the two parts giving different colors in polarized 

 light, according to the position of their optic axes. The crystals polarize in 

 quite brilliant colors, often much resembling quartz in transparency and 

 appearance, the distinction, however, being made by the general rectangular 

 form and the fissures and cleavage lines of the sanidin. In very small 

 and irregularly shaped, often ragged, crystals, sanidin makes up almost 

 entirely the groundmass of many of the rhyolites. These small crystals 

 are only visible in polarized light, being seen with tolerable distinctness 

 when the nicols are crossed. 



Plagioclase, or oligoclase, occurs quite frequently, but always subordi- 

 nate to the sanidin. It can be easily recognized under the microscope by 

 its characteristic colored banding in polarized light, caused by the twin 

 lamellation of the triclinic crystals. In some cases when this is not so 

 distinct, dependence must be placed upon the more weathered or decom- 

 posed condition of the smaller, needle-shaped crystals. Plagioclase crystals 

 often interpenetrate those of sanidin, the two forming together a symmetri- 

 cal crystal, one end being sanidin and the other plagioclase. It is also seen 

 as minute crystals, inclosed in large sanidins. Inclusions are less frequent 

 than in sanidin. 



* Zirkel, in the Microscopical Petrography of the Survey of the Fortieth Parallel, p. 197, mentions 

 the occurrence of a fluid inclusion in rhyolitic quartz, hut concludes that the quartz itself is a foreign 

 inclusion in the rhyolite. 



