218 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



of altered olivine is well brought out by the serpentine and iron ore. In 

 the centers of the meshes there remain small masses of a felt of tremolite 

 needles (pilite). This alteration of the olivine coi-responds to that first 

 described by Lewis/ and more recently by Professor Boimey and Miss 

 Raisin,^ from a rock — kimberlite — very similar to the picrite-porphyries 

 here described. He writes as follows: "It frequently happens that while 

 serpentinization begins at the outside of a crystal, fibrous tremolite begins 

 growing within, finally forming- a mass of asbestiform fibers surrounded by 

 a zone of green .serpentine." 



The minerals which composed these black picrite-porphyries Avere the 

 same as those constituting the gray ones. These minerals were olivine, 

 pyroxene, hornblende, biotite, magnetite, and ilmenite. They were cemented 

 by a glass matrix. The glass is completely altered. All of the minerals 

 are represented by pseudomorphs. Remnants of the original hornblende 

 and biotite alone are preserved. 



The contours of the original pyroxene crystals are filled with pilite, 

 serpentine, and mag-netite. The serpentine is present in greater quantity 

 in these pyroxene pseudomorphs than it was in the pyroxene pseudomoi'phs 

 in the gray picrite-porphyries. The alteration of the hornblende results 

 in the production of an aggregate of chlorite inclosing grains of calcite, 

 some sphene, and iron oxide, similar to that in the gray picrite-poi'phyries. 

 The biotite, magnetite, and ilmenite also show those characters which have 

 been described for the same minerals in the first-described picrite-porphyries. 



Between all of the foregoing minerals we find a fine felty chlorite 

 mass containing grains and dendritic masses of iron ore and a few needles 

 of tremolite. This corresponds to the material forming the cement for the 

 minerals in the gray porphyries, and, like that, is believed to represent an 

 original vitreous matrix. 



In one of the dark picrite-porphyries the magnetite is present in large 

 quantity and is very noticeable, crystals of it standing out upon the weath- 

 ered surface. This rock did not affect the magnetic needle very powerfully, 

 though it was expected that it would do so. However, another one of 



' Ou a diamondiferous peridotite and the genesis of the diamond, by H. C. Lewis: Geol. Mag., 

 3d ser., Vol. IV, 1887, p. 22. 



Papers and notes on the genesis and matrix of the diamond, by the late Henry Carvill Lewis, 

 edited by Prof. T. G. Bonney, Loudon, 1897, p. 14. 



'-' Notes on the diamond-bearing rook of Kimberly, South Africa, Part II, by Prof T. G. Bonney 

 and Miss C. A. Raisin : Geol. Mag., 4th series, Vol. II, 1895, p. 496. 



