560 A TREATISE ON METAMORPHISM. 



into the belt of cementation, and maj even pass into the zone of anamor- 

 phism. It may be metamorphosed in those positions, and produce rocks 

 different in character from ordinary metamorphosed sedimentary rocks, 

 because the material has a different chemical composition. It is pointed out 

 in another place (see pp. 831-833) that the jaspilites of the Lower Huronian 

 of the Marquette district represent a weathered belt, which has been 

 overridden by the Upper Huronian seas, deeply buried by later sediments, 

 and metamorphosed under the conditions of the zone of anamorphism. 



TRANSITION BETWEEN BEET OF WEATHERING AND BELT OF 

 CEMENTATION. 



Before taking up " The belt of cementation," it will be well to call atten- 

 tion to the fact that the belts of weathering and cementation are not sharply 

 separated, but there is a transition between them. In many cases the 

 explanation of the transition is partly that locally strong downward currents 

 carry solution as a preponderant process well below the level of ground 

 water, and partly that a considerable quantity of oxygen may be carried 

 some distance below the level of ground water and produce reactions charac- 

 teristic of the belt of weathering. In other cases the level of ground water 

 rises and falls, as explained on pages 423-429, and therefore there is a 

 belt in which the conditions are alternately those of the belt of weathering 

 and the belt of cementation. While in general the transition from one belt 

 to the other is somewhat gradual, in some instances it is rather abrupt. An 

 excellent illustration of an abrupt change is that given by Culver in the 

 case of the diabase in Minnehaha County, S. Dak. ffi This rock is thoroughly 

 disintegrated and apparently much decomposed to a depth of 6 or 8 meters, 

 that is, to the bed of the stream which marks the limit of ground water. 

 Says Culver: "The limit of decomposition seems to be marked by the 

 position of the stream; the rock in its bed is firm and apparently unaltered." 



As an illustration of a transition belt of considerable width may be cited 

 the iron-ore deposits of the Lake Superior region. Many of these deposits 

 extend from the surface to a depth of 200 to 500 or more meters, but at the 

 greater depths they usually become gradually smaller and less rich in 

 hematite. The level of ground water is rarely deeper than 30 meters. 



" Culver, G. E., and Hobbs, AVilliain H., On a new occurrence of olivine-diabase in Minnehaha 

 County, South Dakota: Trans. Wisconsin Acad. Sci., Arts, and Letters, vol. 8, 18SS-1891, pp. 206-207. 



