58 KEMP'S ORE DEPOSITS. 



<?ases, have occasioned impregnations and disseminations, even when 

 their character is obscured. The cracks need be but small and nu- 

 merous to have occasioned far-reaching results. If a fault fissure, 

 as a possible conduit of supply, crosses the axis of the fold, the 

 necessary conditions are afforded for extended horizontal enrich- 

 ment. Recent explorations with the diamond drill at Mine la Motte 

 seem to corroborate such an hypothesis. Should the anticline 

 or roll afterward sink toward the horizontal, a very puzzling de- 

 posit might originate. Shear zones have been already discussed at 

 length (1.02.03), as have true veins and volcanic necks (see also 

 2.09.20). As regards contact deposits, the igneous rock, which 

 usually forms one wall, may serve two different purposes. It may 

 act merely as an impervious barrier which directs solutions along its 

 course, or serves to hold them, either because it is itself bent into 

 a basin-like fold, or because it forms a trough with a dense bed 

 dipping in an opposite direction. Such relations occur in the Mar- 

 quette and Gogebic ranges of the Lake Superior iron region. It is 

 not apparent that in these cases the igneous rock has in any degree 

 stimulated circulations. In the more characteristic " contact depos- 

 its " the igneous rock has apparently been a strong stimulator of 

 ore-bearing circulations, and often the source of the metals them- 

 selves. This form of deposit becomes, then, an attendant phenom- 

 enon, or even a variety, of contact metamorphism. Under 11 

 chromite is the chief illustration. The mineral is practically limit- 

 ed to serpentinous rocks, and is distributed through them in irreg- 

 ular masses. It appears to be an auxiliary product of alteration. 



1.06.20. III. The debris that results from the weathering of 

 rock masses under the action of frost, wind, rain, heat, and cold is 

 washed along by the drainage system of a district, and the well- 

 known sorting action transpires, which is so important in connec- 

 " ; on with the formation of the sedimentary rocks. Minerals of 

 great specific gravity tend to concentrate by themselves, while 

 lighter materials are washed farther from the starting point, and 

 settle only in still water. Stream bottoms supply the most favor- 

 able situations, and in their bars are found accumulations of the 

 heavier minerals which are in the surrounding rocks. The com- 

 monest of these are magnetite, garnet, ilmenite, wolframite, zircon, 

 topaz, spinel, etc., and with these, in some regions, native gold, 

 platinum, iridosmine, etc.; in other places cassiterite, or stream tin, 

 as described under tin. Even an extremely rare mineral such as 

 monazite may make a sandbar of considerable size. (See O. A. 



