332 



DYNAIVUCAL GEOLOGY. 



iron, also on oxidizing gives red or yellow-brown colors to the decomposed 

 material of the rock. Moreover, the percolating waters carry the changes 

 downward, and especially along the sides and center of the vein. The waters 

 descending along the walls of the vein (and any ascending vapors also) often 

 alter the adjoining rock to clay, making along the side walls the selvage of the 

 vein. Further, the waters may carry along carbonic acid, or sulphuric acid 

 (made from oxidized sulphur), and so convert oxidized metals — as of lead or 

 copper — into carbonate and sulphate. In this way phosphates, arsenates, and 

 other salts of the metals, as well as carbonates and sulphates, become mixed 

 with the ores of the vein as secondary products. 



2. Structure. — Fissure veins are either simple or banded. Those simple 

 in structure are alike in mineral or minerals from side to side ; while the 

 banded have the materials in layers parallel or nearly so to the walls, so that 

 in a cross-section they look banded. 



Granitic or feldspathic veins are usually simple. They may have great 

 width, extending sometimes to 100 feet, and may consist of a number of 

 minerals ; but the minerals are not ordinarily arranged parallel to the walls. 

 The larger veins sometimes contain feldspar and quartz in crystalline masses 

 that weigh tons, and mica in plates a yard across, and occasionally beryls as 

 large as a flour barrel. A beryl of Grafton, IST. H., weighed 2900 pounds. 

 Some spodumene crystals are four feet long. From this extreme magnitude 

 there are gradations to those in which the crystallizations are an inch and 

 less in size. The granite of veins seldom has the moderate fineness and even- 

 ness of grain fitting it for architectural purposes ; the even-grained kinds are 

 probably always of igneous origin. 



305. 



6b 43212 4 56 



306. 



307. 



m^ 



"ill 



'll 



I'M 



/'': 

 'll 



I ll 



/I 



m 

 ■if 



li'inlj'i , 



Banded vein, Valpa- 

 raiso, D. 



IVI 



V'i 



IJ,!,' 



Quartz vein, Cheshire, 

 Conn. D. 



Banded vein, at Godolphin Bridge, 

 Cornwall. De la B. 



But granitic veins are sometimes banded, as in Fig. 305, in which 1, 3, 

 and 6 are bands of quartz ; 2 and 4, bands having the structure of gneissoid 

 granite, and 5 that of gneiss. Fig. 306 contains a two-banded vein of quartz. 

 It illustrates the usual mode of origin of bands, showing that they are layers 

 made by deposition against the two walls. It is also a vein of copper ore, 

 the ore lying in the wider open portions of the vein. 



Figs. 307-309 represent other banded veins, having the bands in two 



