Means based on Table 51, illustrating the control of Iva by fineness and width.. 53. Comparison of synthetic indexes for capacity and mean velocity, under the several conditions of constantdischarge, constant depth, and constant slope....

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HYDRAULIC-MINING DEBRIS IN. THE SIERRA NEVADA.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 131.238.16.30 on Wed, 13 Aug 2014 16:32:36 UTC All use subject to JSTOR Terms and ConditionsABSTRACT Many closely spaced kimberlite diatremes are characterized by major differences in mineral composition of their assemblages (both kimberlite and inclusions). Although some of this variation clearly is a function of local differences of host rock through which the upward moving magma passed during emplacement of the respective pipes, variations in type and intensity of alteration are probably more significant to an understanding of emplacement history. The Sloan 1 and 2 pipes in the northern Colorado-southern Wyoming kimberlitic diatreme field are approximately 150 m apart and contain substantially different assemblages especially of nodular material. These pipes are evaluated in terms of emplacement related alteration processes. Intensity and type of secondary reaction may be a function of time, magnitude, and/or level of intrusive activity. Pipes that fail to penetrate uppermost crustal levels would approximate "roofed" dikes of similar composition in which fresh upper mantle and lower crustal materials are preserved, although carbonated and fenitized nodules may be present. Diatremes that break through to the surface are characterized by variably altered constituents (serpentinized, fenitized and carbonated), the intensity of alteration increasing with magnitude and duration of intrusion. If magma intrudes upward along essentially parallel, closely spaced paths, penetration generally will be more effective along conduits of maximum volume or minimum host rock resistance (e.g., intersecting faults). Pressure reduction accompanying breakthrough of a primary conduit will essentially terminate or cut off further upward migration or intrusive activity in subsidiary conduits. Applicability of the "cut off" mechanism of pipe emplacement for explaining major differences in style of intrusive activity is evaluated by comparison with an experimental laboratory model.

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