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THE BASIC MASSIVE ROCKS, ETC.
705

transparent and almost colorless. In thick pieces a yellowish green tinge may be noticed, but in thin slices no recognizable tint may be detected. The inclusions are opaque dendritic particles, spongy magnetite, and secondary products, among which may be mentioned yellowish serpentine, chlorite, and opaque and yellowish-brown earthy substances. These may occasionally entirely replace the original mineral, but more frequently they occur only in the cleavage and other cracks in the fresh olivine, or along its edges.

In most cases the olivine is so fresh that it was thought worth while to have an analysis of it. This has been made by Mr. Hillebrand, who had furnished him a powder consisting of beautifully fresh olivine intermingled with a little diallage, the mixture having been separated from rock No. 8589 by means of methylene iodide. The olivine was isolated by digestion with hydrochloric acid, and the solution obtained was analyzed with this result:

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO CoO NiO CaO MgO H2O Total.
35.58 1.22 .92 tr. 33.91 .35 .20 ? .90 26.86 .31 100.25

The olivine is thus a hyalosiderite with Mg:Fe about 1½:1. The small quantities of manganese and cobalt present in it are of interest from the point of view of Sandberger,[1] as affording another indication that olivine is frequently that constituent of a rock which is the source of the material for ore segregations. In the present instance they are of little significance, however, since so far as known the only ore occurring within the large areas covered by the basal gabbro are magnetite and ilmenite. At Copper Lake, in Secs. 9 and 10, T. 64 N., R. 4 W., weathered masses of the gabbro are stained with a green coating of malachite, and the same[2] staining has been noticed at the contact of the Pigeon Point gabbro with a red granophyric rock, where it has resulted from the alteration of chalcopyrite, but in neither case is the copper compound in sufficient quantity to constitute an ore.

  1. ↑ Cf. J. F. Kemp: A Brief Review of the Literature of Ore Deposits.School of Mines Quarterly, XI., No. 4, p. 366.
  2. ↑ Bull. U. S. Geol. Survey, No. 109.
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