14 KEMP'S ORE DEPOSITS. 



1.02.03. A phenomenon which is especially well recognized in 

 metamorphic regions, and which is analogous to those last cited, 

 is furnished by the so-called " shear zones." A faulting move- 

 ment, or a crush, may be made apparent by changes in mineralogi- 

 cal composition and structure. Massive diabases, for instance, 

 pass into hornblende-schists or amphibolites for limited stretches. 

 Garnets and other characteristically metamorphic minerals ap- 

 pear, and pyroxenes alter to amphiboles. Strains are manifested in 

 the optical behavior of the minerals in thin sections of specimens 

 taken from such localities. These crushed strips, or shear zones, 

 may be formed with very slight displacement, but they afford 

 favorable surroundings for the formation of ore bodies. This con- 

 ception of the original condition of a line of ore deposition is a 

 growing favorite with recent w r riters, and combined with the idea 

 of replacement is often applicable. (See Example IV, Butte, 

 Mont.) Fahlbands, which are very puzzling problems, may have 

 originated as shear zones. 



1.02.04. A more extended effect is produced by the mono- 

 cline, which has a double line of shattered rock marking both the 

 crest and the foot of its terrace. Anticlines and synclines occasion 

 the greatest disturbances. Comparatively brittle materials like 

 rocks cannot endure bending without suffering extended fractures. 

 When strained beyond their limit of resistance, along the crest of 

 an anticline and in the trough of a syncline, cracks and fractures 

 are formed which radiate from the axis of each fold. As these 

 open upward and outward in anticlines, they become the easiest 

 points of attack for erosion, so that it is a very common thing to find 

 a stream flowing in a gorge, which marks the crest of an anticline, 

 while synclinal basins are frequently left to form the summits of 

 ridges, as is so markedly the case in the semi-bituminous coal basins 

 of Pennsylvania. It is quite probable, however, that the anticline 

 may have been leveled off at this fissured crest because it was up- 

 heaved under water and became exposed at its vulnerable summit 

 to wave action. 



Ore deposits may collect in these fissured strips, of which the 

 lead and zinc mines of the upper Mississippi Valley (Example 24) 

 are illustrations. Such fissures are peculiar in that they exhibit no 

 displacement. The accompanying figure is from a photograph of 

 a ^gaping crack in the Aubrey (Upper Carboniferous) limestone, 

 twenty-five miles north of Caflon Diablo station, Ariz., on the At- 

 lantic and Pacific Railroad. It was caused by a low anticlinal 



