17 



(3) Veins may pinch together by the creeping of the strata of 

 the wall. In such cases the walls are mashed and the veins 

 filled in p:irt at least by the pressure of the superincumbent weight. 



(4) Veins may widen out and rise to lens-shaped ore masses 

 within the vein (Fig. 15). 



(5) They may also be made irregular by repeated crustal 

 movements, which break the rock into rubble-like material. 

 The filling of these incipient fracture planes gives rise to the brec- 

 ciated veins, as shown in Fig. 16. 



(6) Irregularities are also formed by the solution of limestones 

 by percolating waters, charged with carbon dioxide. 



Granite. 



\ltereaf Schortoceous 

 Granite. 



Peach with Cassiteritt 

 ombs of Quartz. 



Siliceous Iron Ore 



Combs of Quartz 



Peach with Cassittrite. 



Altered Schorlaceous 

 Granite. 



Granite. 



FIG. 17. Structure of a lode at the Bellau mine, St. Just, Cornwall, Eng- 

 land. (After Thomas and MacAlister's Geology of Ore Deposits.) 



Ribbon Structure. A banded or ribbon structure is not uncom- 

 mon in the veins. In fact, according to LeConte, it is as common 

 in veins as the columnar structures is in dikes. The layers upon 

 the two sides usually correspond with each other in color or in 

 composition, and, therefore, gives rise to a beautiful striped ap- 

 pearance. Sometimes these successive layers are of different 

 2 



