372 GEOLOGY OF THE EUREKA DISTRICT. 



indicating auorthite, which is the feldspar so abundant in the pyroxene-andesite. 

 The next variety is a still more glassy rock, 55; it is a colorless glass with a pearlitic 

 fracture, with scattered microlites, which are beautifully developed, some, in long 

 prisms with pyramidal termination and transverse jointing, appear to be apatite; 

 others, shorter and stouter, are more doubtful, but resemble those in 53, which are 

 probably augite. There are also curved and tapering microlites and strings of grains 

 apparently of the same mineral. Larger microscopic crystals scattered through the 

 groundmass are hypersthene, hornblende and biotite. There is but little magnetite. 

 Of the macroscopic crystals, feldspar is very abundant as labradorite, with possibly a 

 little anorthite; biotite is also abundant, and hornblende and hyperstheue are scarce. 

 There is one rounded grain of quartz with good rhombohedral cleavage. Thin section 

 75 is like 55; 75 is taken from the ^ame specimen and shows a slight modification 

 caused by streams of opaque particles and hair-like trichites, which lie scattered or 

 aggregated in the most delicate dendritic forms. A small part is black in incident 

 light and may be magnetite, but the greater part is bright red and is hematite. Thin 

 section 74 is similar. 



The remaining varieties differ from the preceding in having the glass base tilled 

 with opaque and more or less transparent, ill denned microlites and flocculent matter, 

 imparting to it a black, led, yellow or white color. Thin section 56 is from a brecciated 

 pearlite rich in angular fragments and crystals of labradorite, hypersthene, augite, 

 hornblende, and magnetite, with no mica. The groundmass is glass, probably o 

 itself colorless, but so crowded with microlites and more or less opaque grains as to 

 appear in the section dark brown, yellow, and bluish. In some places it is brown and 

 globulitic, in others it is tilled with flocculent matter, which is brown in transmitted 

 light and white in incident; in other places it is colorless, with few microlites. The 

 transition from one kind to another is generally sudden and the flow structure is well 

 ^narked, being especially beautiful in thin section 57, which is similar to 56, as is also 

 64, though of a lighter color. -Mica and quartz are both wanting in these last three 

 thin sections. The varieties represented by thin sections 58 and 66 are very similar 

 to the last, much more so than their appearance in the hand specimen would indicate. 

 Their secretions are the same labradorite, hyperstheue, and hornblende, with a little 

 augite and no mica or quartz. They are not brecciated, however, and the groundmass 

 is lighter colored, the opacite being red and white in incident light and the flow 

 structure very striking. There is a tine example of partially altered hypersthene 

 shown in Fig. 9, PI. in. 



Variety 59 has a more pumice-like groundmass, the glass having numerous gas 

 bubbles. It is much lighter colored, with more white and less yellow opacite, and is 

 in part cryptocrystalline. The pheuocrysts are labradorite,, eight-tenths of the feld- 

 spars showing stria} and the rest probably belonging to plagioclase. There is a very 

 little partially altered hypersthene, considerable hornblende, and much biotite. The 



