CHAPTER V. 



ON CERTAIN STRUCTURAL FEATURES OF MINERAL VEINS. 



1.05.01. Banded Structure. Mineral veins sometimes exhibit 

 a banded structure, by which is understood the arrangement of 

 the ore and gangue in parallel layers that correspond on opposite 

 walls. They are most conspicuous where the walls are well de- 

 fined. The solutions which have brought the minerals have varied 

 from time to time, and the precipitated coatings correspond to 

 these variations. They alternate from gangue to ore, it may be, 

 several times repeated. The ore may be in small scattered masses 

 preserving a distinct lineal arrangement in the midst of the barren 

 quartz, calcite, barite, fluorite, siderite, etc., or itself be so abun- 

 dant as to afford a continuous parallel streak. The commonest 

 ores so observed are pyrite, chalcopyrite, galena, blende, and the 

 various sulphides of silver. The veins of the Reese River district, 

 in Nevada, furnish good illustrations of alternating ruby silver 

 ores and quartz. Those of Gilpin County, Colorado (Example 

 1VO), afford alternations of pyrite, chalcopyrite, and gangue. (See 

 figures in Endlich's report, Hayderfs Survey, 1873, p. 280.) The 

 Bassick Mine, in Colorado, has pebbles remarkably coated. The 

 figure on p. 36 shows a vein at Newman Hill, near Rico, Colo. 



Banded veins, however, except of a rude character, are not com- 

 mon in this country. They have received much more attention in 

 Germany, where, especially near Freiberg, they show remarkable 

 perfection. The famous Drei Prinzen Spat Vein, figured by Yon 

 Weissenbach and copied in many books, has ten corresponding al- 

 ternations of six different minerals on each wall. 



1.05.02. A line of cavities, or vuggs, is often seen at the cen- 

 tral portion of a vein, into which crystals of the last formed miner- 

 als emerge, forming a comb (see p. 36). These may project into 

 each other and interlock, especially if quartz, forming a comb 

 in comb. The same may occur between side layers. These cavities 

 are a most prolific source of finely crystallized minerals. If, after the 



