petro(iJ:aimiv— i:ii\(>i.i ri:s and ii.'aciivtks. 4<)1 



not vory tniiispareiit and not at all dicliroitic (I'lalc II, l''i«^. 4). The pris- 

 matic sections are deeply striated, (putc transparent, and have a h"^ht yellow- 

 ish-brown color. The crystals are often partially decomposed, becoming- 

 ([inte ragged, broken and opacpie and full of magnetite cr}'stals and grains, 

 which latter form a border comj)letely aronnd the edge of the crystal (Plate 



I, Fig. 2). This, when carried further, changes the biotite crystal into an 

 aggregation of magnetite grains in the form of biotite, of which only an 

 occasional transparent particle remains to show the origin of the mass. 

 Biotite generally occurs in trachyte in large crystals arid not in small scales 

 or leaves. 



Magnetite occurs in g-reat abundance as grains, large masses and shar|) 

 crystals. 



^linute, colorless needles of apatite have been noticed in all trachytes, 

 almost without exception, being most readily found when included in the 

 large crystals of sanidin or hornblende. It may therefore be regarded as 

 a constituent of the rock, although in minute quantities, and only discover- 

 able in the thin section under the microscope. In the Black Hills rocks, it 

 is of frequent occurrence. 



Zirkel reports tridymite to be a distinct ingredient of some trachytes, 

 in the form of small six-sided scales or leaves, arranged in groups some- 

 what as tiles or slates on a roof, but none was satisfactorily determined to 

 be present in these rocks 



Titanite and augite are occasionally noticed, but were not found in 

 the rocks under examination. 



The groundmass, as seen under the microsco})e, is generally cpiito 

 crystalline, being usually made up of thickly crowded feldspar microlites, 

 with needles and grains of green hornblende, and magnetite grains. A 

 fluid-like structure is often recognized from the position of the hornblende 

 crystals, which ap})ear as if imbedded in a mass flowing in streams around 

 a large sanidin crystal or i>rain of niao-netite. There are several fine 

 examples of this structure among the trachytes [128] and [159-162] (Plate 



II, Fig. 3). The number of the felds})ar microlites can onlv be truly ap})re- 

 ciated by turning the slide on its center between crossed nicols, in order to 

 cause all the crystals successively to become bright by changing their 



