THE BASIC MASSIVE ROCKS, ETC. 70 5 



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 par- 

 ticles, 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 beauti- 

 fully fresh olivine intermingled with a little diallage, the mixture 

 having been separated from rock No. 8589 by means of methy- 

 lene iodide. The olivine was isolated by digestion with hydro- 

 chloric acid, and the solution obtained was analyzed with this 

 result : 



SiO s Ti0 2 Alg0 3 Cr s O a FeO MnO CoO NiO CaO MgO H s O Total. 

 35.58 1.22 .92 tr. 33.91 .35 .20 ? .90 26.86 .31 IOO.25 



The olivine is thus a hyalosiderite with Mg : Fe about 1^: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 ores occurring within the large areas cov- 

 ered by the basal gabbro are magnetite and ilmenite. At Copper 

 Lake, in Sees. 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. 



J Cf. J. F. Kemp : A Brief Review of the Literature of Ore Deposits. School of 

 Mines Quarterly, XL, No. 4, p. 366. 

 2 Bull. U. S. Geol. Survey, No. 109. 



