ORE DEPOSITS 371 



one side has no corresponding layer on the other. A banded struc- 

 ture may also be brought about when, as a result of movements 

 which rend the vein from one of its walls, the fissure is reopened 

 and minerals are subsequently deposited in the cavity thus formed. 

 Several such movements may 

 take place and two or more 

 bands may be formed. In 

 some veins the filling consists 

 wholly or in part of broken 

 rock (Fig. 357), the spaces 

 between which are filled with 

 quartz or other minerals. 



Form and Extent of Veins. 

 — The form of veins usually Fig. 357. —A section showing a fault breccia, 

 depends upon the shape of Such breccias have sometimes been cemented 

 1 r i_« i_ ^t rn together by precious minerals and are valuable 



the fissures which they nil, 1 -1 r\c^ r>- j w , \ 



J ' ore deposits. (Alter Ries and Watson.) 



and their width, length, and 



depth consequently vary greatly. Some are only a fraction of an 

 inch wide, while others are 200 or 300 feet in width. The length 

 is even more variable, being in some cases 50 or more miles and in 

 others only a few feet. Some veins have been followed to a depth 

 of more than 5000 feet, while others have disappeared a few feet 

 beneath the surface. 



Source of Vein Material. — It has been shown by chemical analysis 

 that nickel, copper, tin, lead, and other metals occur in minute 

 quantities in both sedimentary and igneous rocks, and it is generally 

 believed that the ores which are now concentrated in veins were 

 originally disseminated through the rocks; that they have been dis- 

 solved out by water, carried to fissures or other cavities and there 

 deposited. This theory is borne out by the fact that in certain 

 places veins are actually being formed at the present time by deposi- 

 tion from water. For example, near Boulder, Montana, a hot 

 spring is depositing gold-bearing quartz identical with the gold and 

 silver-bearing quartz veins of the region. Steamboat Springs, in 

 Nevada, are strongly alkaline and are depositing quartz in fissures 

 and thus forming veins. Sulphides of iron, lead, mercury, and zinc 

 are found in recently filled fissures. 



The water which acts as a transporting agent is either meteoric 

 (rain water) or magmatic (the waters issuing from cooling masses 

 of rock). The latter are believed by many geologists to be the more 



