UNSTRATIFIED CONDITION. 



123 



and 5, of gneiss, 

 three bands, one 





Fig. 



135 



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65 4321 2 4 5 





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bands of quartz ; 2, 4, of a gneissoid granite ; 

 Of banded veins, the simplest is a vein with 

 central; but the number may be a score or 

 more. 



Instead of being simply rock-material, as in 

 fig. 132, the bands may be partly metallic ores 

 of different kinds, and calcite, heavy spar, fluor 

 spar, may make the alternating bands instead 

 of granite or gneiss. A great vein at Frei- 

 berg consists of layers of blende, quartz, fluor 

 spar, pyrites, heavy spar, calcite, each two or 

 three times repeated, the layers nearly corre- 

 sponding on either side of the middle seam. 



Thus this banded structure is as much cha- 

 racteristic of veins as the columnar structure is of dikes : each 

 fails of the peculiarity in their simpler kinds. 



The bands of a vein are far from uniform at different heights, 

 even when the width of the vein is constant ; and they vary exceed- 

 ingly through the contractions and expansions which take place at 

 intervals. The expanded portions may alone be banded, or consist 

 of layers parallel to the sides, or contain ore. 



The mineral or rock-material accompanying the ore in a vein is 

 called the vein-stone, or gangue. The most common kinds of vein- 

 stone are quartz, calcite, barytes, and fluor. 



In studying veins, besides noting their extent, mineral cha- 

 racter, and structure, it is important to ascertain their strike and 

 angle of dip. There is generally an approximate uniformity of 

 strike in a given region ; and frequently the direction is parallel to 

 the principal line of elevation in the region. The nature of the 

 walls or adjoining rock, and systems of faults, are other points that 

 should receive close attention. 



134. False veins. — Besides the veins and dikes described, there 

 are also false veins. These false veins are fissures filled by sand 

 or clay from above. They are readily distinguished by the sedi- 

 mentary nature of the material ; for all true dikes or veins 

 are occupied by crystalline rocks or minerals. In a similar 

 manner earth and organic remains may be washed into caverns 

 or any open spaces in rocks, and so make, in the very body 

 of an old record, a false entry. Such a conjunction of com- 

 paratively modern fossils with more ancient may lead to error, 

 unless the facts are carefully studied and the true explanation 

 ascertained. 



