1S75.] 4:1D , [Frazer. 



The surfaces of both feldspar and pyroxene (and especially of the 

 latter) are covered with small cavities. 



Plate III. 



Fig. 1. This is a dolerite from Logan's, a shaft contiguous to the 

 Mumper shaft. Besides exhibiting the relations of the light-colored slabs 

 of labradorite to each other, and the pyroxene which forms a matrix 

 for them, there are two distinct apatite crystals reproduced in the print. 



a. Labradorite. 



6. Pyroxene. 



c. Apatite. 



Central black spot, Magnetite. 



Fig. 2. Thin section of a dolerite from a shaft on Mumper's property 

 about 1 mile N. of Dillsburg. The dyke of which this is a section cuts 

 the ore bed at a short distance beneath the surface. 



In this print there is a labradorite of unusual size, in which is im- 

 bedded a small mass (of pyroxene) (?) which appears black in this light. 

 The sbriation of other labradorite crystals is distinctly seen, while the 

 outlines of the magnetite crystals are unusually sharp. 



a. Labradorite. 



5. Pyroxene. 



c. Magnetite. 



Plate IV. 



The figures in this plate wex-e photographs of the same object but 

 under different conditions of polarized light. Figs. 1 to 5 inclusive, 

 were photographed in five different positions of the analyzer. A peculiar 

 crystal of pyroxene which exhibits a median line differing in color from 

 the body of the crystal was made the guide. The purpose of these ex- 

 periments was to see whether means could not be discovered to di^crimi- 

 nate between the effects of anactinic light and opacity, by the camera 

 alone. The object was a thin section of a dolerite from Beeler's farm, 

 2 miles S. W. of York, marked No. 4. 



Fig. 1. This pyroxene appears of a light color and with a dark core, 

 which in turn contains an irregularly formed light-colored axis. The 

 boundary between this crystal and the magnetite at its right hand extrem- 

 ity is sharply defined ; and the division between this pyroxene and a 

 neighboring fragment just below its lower edge is also evident. 



Fig. 2. In this photograph polarizer and analyzer are in the same 

 phase. The main crystal is still li^ht-colored, but there is less defini- 

 tion about the middle part of its dark nucleus, a light band extending 

 nearly across it at this place. The pyroxene lying below its lower edge, 

 which was dark in Fig. 1, has now become light, and the line of division 

 between the two crystals is nearly obliterated, except at one point whei-e 

 a small magnetite appears in relief against the light background. The 

 angle of the analyzer was not determined. 



