398 GEOLOGY OF THE EUREKA DISTRICT. 



PLATE IV. 



1. FRACTURES ABOUT GLASS INCLUSIONS IN QUARTZ OF RHYOLITE (CROSS SECTION). 



Thin section 111, magnified 300 diameters. A cross section of a quartz crystal that bears glass with gas 

 buhhles in dihexahedral cavities. About each inclusion the quartz is cracked fora short distance 

 in three planes, corresponding to three of the planes of symmetry passing through its vertical 

 axis. The cracks appear as six -rayed stars that are parallel to each other throughout the section. 

 In this figure a number have been brought together from different parts of the same quartz sec- 

 tion, in order to show the different appearances when the inclusions are cut through the middle 

 or near one end, or when the section passes just above or below them. From the inclusion nearest 

 the top of the figure it will be seen that the section is slightly inclined and not exactly at right 

 angles to the principal axis of the quartz crystal. The illustration shows the upper end of this 

 last-named inclusion and the lower end of the one just below it to the right. 



2. FRACTURES ABOUT GLASS INCLUSIONS IN QUARTZ OF KHYOLITE (LONGITUDINAL SECTION). 



Thin section 111, magnified 300 diameters. A longitudinal section of a quartz crystal only 1 millimeter 

 from that in Fig. 1, showing the same kind of glass inclusions with vertical fractures. These 

 are drawn as they occur, without any change of position. Those lying at the surfaces of the 

 quartz section have had the gas bubbles cut in grinding and filled with balsam. 



3. QUARTZ-CONGLOMERATE. 



Thin section 501, magnified 33 diameters. Section of fine grained conglomerate with siliceous cement, 

 showing that the apparently granitoid quartz grains are rounded, water-worn grains, about 

 which the silica of the cement has crystallized with the same crystallographic orientation as the 

 nucleus, thus extending the individual until obstructed by the surrounding fragments. 



4. QUARTZ FRAGMENT IN BASALT OP MAGPIE HILL. 



Thin section 295, magnified 28 diameters. Section of crystalline basalt, showing an irregularly shaped 

 fragment of primary quartz, surrounded by a shell of angite crystals and patches of calcite. 

 The somewhat darker, broader grains in the augite shell are piedmontite. The rock is composed 

 of feldspar, minute augite, and magnetite crystals, with larger crystals of red altered olivine. 



