GEOLOGY OF GRAVES MOUNTAIN 213 



ard* has a thicloiess of more than 300 feet, and "presents numerous in- 

 cluded zones or layers, varying from one to three feet in thickness, in which 

 is found imbedded, masses and crystals of lazulite." The observations of 

 the senior author, which were incomplete from want of time, did not indi- 

 cate the distribution of lazulite along zones or layers in the quartzite, but 

 rather as numerous crystals in single individuals and aggregates of very 

 irregular distribution. The occurrence of the mineral in places sometimes 

 suggested a tendency to form nests and bunches in the rock. 



The lazulite almost invariably occurs as crystals ranging in size up to an 

 inch and more in length, bounded by crystal faces — combination of prisms 

 and pyramids, and in cross section from irregular to squarish and triangu- 

 lar in outline. The crystals are opaque, of deep azure blue color when 

 fresh but of lighter or paler color after prolonged effects of weathering, 

 have uneven fracture and indistinct cleavage, but pronounced vitreous 

 luster. The monoclinic crystals are acute pyramidal in habit, occasionally 

 flattened from extension of the pyramidal faces, and are frequently twinned. 

 Professor Shepardf described and figured five crystals of lazulite from this 

 locality and remarked that a twin (fig. 5) "is by far the most abundant 

 form, equalling in frequency all the others combined." 



The lazulite is frequently intimately associated with greenish to color- 

 less, massive, columnar cyanite partially altered to muscovite, and small 

 red crystals of rutile. Frequent small crystals of rutile are observed in 

 the quartzite by aid of the lens. Weathered surfaces of the rock are quite 

 rough showing characteristic pitting from resistant coars^ cyanite and lazu- 

 lite in relief, with the fine-grained sugary quartz falling out from probable 

 granulation, forming the depressions. Scales of colorless mica are con- 

 spicuous in the weathered specimens. 



In thin sections under the microscope, the lazulite is usually some shade 

 of light blue, with pleochroism varying according to depth of color, it being 

 distinct in the deeper colored sections and weak in the light-colored ones. 

 It has the pleochroism X colorless or nearly so, Y and Z azure blue. The 

 absorption is Z = Y > X. The index of refraction is moderate, double 

 refraction strong; cleavage is indistinct but numerous irregular fractures 

 occur. The axial angle is large being 2 E, = 111° as determined bj^ von 

 Lasaulx. The lazulite contains inclusions of rutile and quartz, and occa- 

 sionally cyanite and muscovite. Alteration has progressed along the 

 periphery and fractures or cracks, yielding very minute scales and fibers 



* Op.cit., 1859. 

 t Opcii., 18.59. 



