110 GEOLOGY OF THE COMSTOCK LODE. 



relations of the feldspars ai'e the same. A horizontal plate of mica shows 

 the interference figure of biotite. The quartzes contain good-sized inclusions 

 of glass and some exceedingly minute ones which appear to be liquid. The 

 groundmass shows a highly fluidal structure. The effect is produced by 

 elongated aggregations of iron ore, embedded in nearly colorless material. 

 This colorless substance appears to be absolutely isotropic in some places, 

 in others it shows pseudo-spherolitic structure, but for the most part it exhibits 

 aggregate polarization as if it were a devitrified substance. In many places 

 it is full of black microlites, which seem to radiate from particles of iron 

 ore. 



Slide 353. 1,700 feet south-southeast of the Amason. 



A third example. — Tliis rock aud sHdc are entirely similar to the preceding; 

 the fluidal structure is less marked than in 304, but the pseudo-spherolitic 

 structure is well developed. The feldspar is mostly orthoclase, and the 

 quartzes with bays of groundmass, etc., contain some glass inclusions, and 

 a very few liquid ones. 



Slide 351. Overman 1142, 200 feet north of Caledonm shaft. 



Specimens tested by Thouiefs method. — A gray rock entirely similar to those 

 already described. Under the microscope it is seen to be somewhat more 

 altered, the feldspars being clouded with calcite. Hornblende and mica 

 occur, and the groundmass shows the same fluidal aud pseudo-spherolitic 

 structure. The quartzes contain more inclusions both of fluid and glass than 

 those of the surface rocks. One of them is of a very unusual character. It 

 is a glass inclusion in a glass inclusion, the inner one bearing a bubble. The 

 inner glass may differ slightly in composition, or may have solidified at a 

 different pressure. This cannot be a case of a cut bubble filled with l)alsam 

 and air, for if the instrument be focused on either surface of the quartz, the 

 inclosure and bubble are out of focus. The inclusion is shown in Fig. 24, 

 Plate III. 



To test the nature of the feldspars in this rock a fragment was pulver- 

 ized and separated in a solution of mercuric iodide in potassic iodide of a 

 specific gravity of 2.65. A large portion of the rock rose to the surface. 



