PETROGRAPHY-EHYOHTE. 511 



resembles both in color and luster [149], being dark-gray, with micro- 

 scopical sanidin crystals and black needles of hornblende. In the thin 

 section under the microscope, it is exactly the same, namely, a crystalline 

 mixture of sanidin, hornblende and magnetite. The sanidins have the 

 same zonal structure as before, as well as the marked difference between 

 the interior and exterior parts of the crystal. The colorless, long, prismatic 

 microlites of apatite are also present and a few masses of plagioclase and 

 biotite. The large, green hornblende individuals are different from those 

 of the scarce, brown and dichroitic biotite, because the former are fresher 

 and much greener than is usual for biotite. Again, if they are biotite, it is 

 remarkable that more of them are not cut parallel to ooP, considering how 

 plentifully they occur in the slide; for all the green, non-dichroitic sections 

 in the above case would be sections parallel to OP, which theory their long, 

 prismatic shape hardly seems to warrant. The biotite crystals also have 

 a border of magnetite grains, which appearance was not observed with 

 the green hornblende, although they include large grains of magnetite. 

 The cleavage lines are very indistinct and difficult to make out, but in a 

 few cases they seem to be decisive. 



The groundmass is, as before, a coarsely crystalline mixture of feldspar, 

 magnetite and small crystals of hornblende. 



The rhyolite [152] from Inyan Kara (?) has a light-gray color and 

 is quite homogeneous in structure, being fine-grained, with numerous 

 very small biotite needles imbedded in the mass, resembling [147] the 

 rhyolite from Inyan Kara. Under the microscope, it seems to be made np 

 of sanidin crystals, with biotite and magnetite in a partially crystalline 

 groundmass. The sanidin is in crystals of moderate size, quite transparent 

 and having the characteristic zone structure, and also twinned crystals. 

 The slender, biotite needles are abundant and generally dichroitic. The 

 difference in the plane of section is strikingly shown in this slide, there 

 being a large hexagonal form of a fine brown color, which shows no change 

 upon a revolution of the nicol, while the prismatic forms change from light 

 to dark-brown. The biotite has magnetite grains through it, as usual in 

 these rocks. Magnetite in small grains of uniform size is quite evenly dis- 

 tributed through the mass, though there is an occasional large piece. The 



