Frazer.] 4:iU [April 18 



OPTICAL properties! 

 Syenite (?) from Cemetery Hill, near Gettysburg. Adams Co., Pa. 



Contains Feldspar, Hornblende and Magnetite, and some Biotite, with 

 Quartz rarely. With a single Nicol's prism, the blades of Hornblende aie 

 fully dichroic. Both that and the feldspar are speckled and spotted. 



Between two Nicol's prisms the Labradorite polarizes through blue, 

 yellow, and lilac ; the Hornblende from white to brown and black ; and 

 the Quartz, which is sparingly present, gives brilliant colors. 



Iq the thick specimen examined under the microscope the feldspar dif- 

 fers from that of the equally thick specimens of dolerite in being more 

 transparent and " icy "-looking, resembling Adularia, and here and there 

 are seen small grains of a transparent mineral giving the rainbow colors 

 of quartz. 



The fine slice reveals the feldspar in a state not easily distinguishable 

 and of weathered appearance, and also several objects, which from their 

 colors, green and red, resemble small fragments of pyroxene. "While 

 therefore, there is no doubt of the occurrence of hornblende in suffi- 

 cient quantity to give the character to this rock, the question as to its 

 proper name will be remanded to future study. 



Dolerite prom Beeler's Farm, 2 Miles S. W. of York. 



This slide at 275 diameters and between Nicol's prisms, shows an ag- 

 gregate of irregular portions of crystals of pyroxene and Labradorite with 

 the accompanying magnetite. The surfaces of the crystals are rough, 

 but they do not seem to be so much affected by weathering as in that 

 inarked No. 3. 



Dolerite (No. 3) from Beeler's Farm, 2 Miles S. W. of York. 



The Labradorite and pyroxene of this specimen, under 275 diameters, 

 appear in much the same condition as those of the slide from the Mum- 

 per dolerite. The blades of Labradorite are twinned and sometimes gen- 

 iculated ; the two individuals polarizing alternately light and brown. 



Certain parts of this slide are very rich in a fine rod-like crystal ap- 

 parently uniaxial which may be set down with safety as apatite. A very 

 large number of these little crystals is distributed throughout the whole 

 mass. 



Dolerite from Mumper Shaft, 1 Mile N. of Dillsburg, York 



Co., Pa. 

 The thin section (magnified 56.8 diameters) and with ^ in. aperture, 

 exhibits blades of Labradorite very finely and regularly striated, mixed 

 together with yellowish green masses of pyroxene irregularly cleft and 

 stippled on the surface like fish roe and containing magnetite, around 

 which is to be seen a brownish-yellow stain due to its partial conversion 

 into ferric hydrate. With appertures of ^^ in., |- in., and 3-16, the same 

 appearances are manifest, but not so clearly. 



