PETROGllArilY—SANIDlN TRACHYTE. 505 



nicols. The deciding peculiarities, tlien, are the form, the dusty or dotted 

 structure, the simi)le refraction, the (huk border and the beliavior of the 

 decomposed portions of the crystals Besides these two large sections, there 

 are quite a number of smaller nosite crystals more irregular in shape, some 

 being hexagonal and others rectangular, more opaque and in varying stages 

 of alteration, though they are still partly isotrope. 



The sanidin is in the form of indistinct, long, rather roughly shaped 

 masses and columnar microlites, lying between the ne[)helite crystals and 

 showing with the hornblende a distinct, curved and fluid-like structure in 

 places, and particularly around the two large nosite crystals mentioned 

 above (Plate I, Fig. 4). The columnar, somewhat fibrous-looking crystals 

 polarize witli tolerable distinctness with crossed nicols. No magnetite nor 

 biotite were found. Besides the minerals already described, no other 

 groundmass is discoverable as the rock is made up of sanidin, ne])helite, 

 and hornblende in intimate union, the structure being quite crystalline. 



The strong gelatinization, large amount soluble in acid, and low silica 

 percentage of this phonolite all aid in confirming the above determinations, 

 and it is to be hoped that more observations of its geological occurrence 

 will be made at some future time. 



The sanidin-trachyte [140] from Black Butte is light-gray, inclining a 

 little to yellow in color, but has not the uniform and fine-grained appear- 

 ance of the rhyolites. Scattered through it, small yellow- spots and dark 

 needles of hornblende can be seen with the loupe. Under the microscope, 

 the section shows the rock to be mostly sanidin, in clear, roughly shaped 

 cr3'stals and partly in masses, with abundant long, prismatic crystals or 

 blades of hornblende. It is quite similar to [124] and [171]. 



The sanidin, besides being the mass of the rock, is also seen in indis- 

 tinct crystals, which have many irregular lines of fissure and also contain 

 long, needle-like, colorless microlites, probably apatite, and hornblende 

 crystals and fragments. Most of it has a cloudy or dusty character, which 

 is seemingly caused by innumerable short, stout microlites of exceedingly 

 minute dimensions scattered in quantities through it. 



The hornblende crystals are of a fine green color, and some have 



