Annual report of the State Mineralogist for the year ending ... . not yet beendeveloped. The Grades of the Ancient Channels.—Owing to small irregularitiesthere is required the development of a considerable length of the chan-nel to determine satisfactorily the average grade. However, disregard-ing the smaller tributaries, the exposed sections show, as a rule, a fairuniformity of grade—certainly as great a uniformity as the modernriver beds. The accompanying longitudinal section of the divide shows the gradesof the summit line, and of the ancient and modern channels, and the depthof erosion (se
Annual report of the State Mineralogist for the year ending ... . not yet beendeveloped. The Grades of the Ancient Channels.—Owing to small irregularitiesthere is required the development of a considerable length of the chan-nel to determine satisfactorily the average grade. However, disregard-ing the smaller tributaries, the exposed sections show, as a rule, a fairuniformity of grade—certainly as great a uniformity as the modernriver beds. The accompanying longitudinal section of the divide shows the gradesof the summit line, and of the ancient and modern channels, and the depthof erosion (see Fig. 9). The course of the ancient river was somewhat more sinuous than thatof the ridge line, hence the apparent grade in the section is somewhatgreater than the actual grade. The grades are given in the form of thenatural sine of the slope angle. The average grade of the ridge line , or one hundred and twenty-seven feet to the mile; that of theancient channel .015, or seventy-nine feet to the mile. 444 REPORT OF THE STATE MINERALOGIST i>loclp$[. The grade line of the deep modern channel is curved to a markeddegree; that of the shallower ancient channel to a scarcely perceptibledegree. In the following diagrams (Fig. 10) there are given the courses andgrades of various patches of channel exposed in mining. The magneticbearings of the down-stream courses are platted from a common point. The first group represents the first period; the second group, the secondperiod. The Paragon grade is exceedingly irregular, the channel bottom risingand falling alternately on the down-stream course. The grades of theDam, Mitchell, Rainbow, and Dix Mines, belonging to the second group,though determined, were not included in the diagram. They are asfollows: Dam and Mitchell, south 50 degrees east .005; Rainbow andDix, south 40 degrees west .005. ANCIENT RIVER BEDS OF FOREST HILL DIVIDE. 445 IV GROUP.
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Keywords: ., bookcentury1800, booksubjectgeology, booksubjectminesandmineralr
