20 O. A. Derby — Manganese Ore Deposits 



quartzite, but which on examination proves to be composed 

 almost exclusively of a iinel y granular mass of ashy white man- 

 ganese garnet. A complete series of alteration phases between 

 perfectly typical garnet rock and merchantable ore can be read- 

 ily selected, and there can be no doubt that the latter results 

 from the decay and leaching of the former. 



In part from the ore body itself, in part from the dump heap 

 of rejected material, the following phases of the garnet rock 

 were obtained. 



1st. A very fine-grained, compact and finely jointed rock of 

 bluish gray color with partings lined with asbestus. Under the 

 microscope the rock is seen to be composed almost exclu- 

 sively of closely appressed idiomorphic grains of white garnet 

 showing a clear border but with the center highly charged with 

 a fine black opaque powder that appears to be graphite. In the 

 somewhat rare interspaces between the garnet grains and 

 molded upon them is a glassy white anisotropic mineral gen- 

 erally altered to a mass of asbestiform fibers which, so far as 

 its form, optical properties and cleavages can be made out, is a 

 member of the amphibole group. A qualitative test on a small 

 amount of material, largely asbestiform, separated with heavy 

 liquids shows the presence of silica, lime, magnesia, iron, man- 

 ganese and alumina, the latter in scarcely more than traces, 

 thus confirming the identification of the mineral as an alumina- 

 free amphibole. The only accessories that could be detected 

 in the section are apatite in rare grains and the above mentioned 

 inclusions in the garnet, the reference of which to graphite is' 

 apparently confirmed by a distinct reaction for carbonic acid 

 after fusion with nitre and by the presence of free graphite in 

 other similar rocks from the same ore body. An analysis of 

 this rock kindly made by Dr. G. Florence gave : 



SiO, 38-47 



A1 2 3 21-07 



Fe 2 0, ..__ 7-38 



MnO 27-90 



CaO 4-70 



99-52 



A second specimen of the same general type but not jointed 

 is largely oxidized and stained at the margins and in patches 

 with manganese oxide and has the amphibolic mineral stained 

 brown while the apatite is much more abundant. 



2d. A dark brown rock heavily charged with manganese 

 oxide and too friable to permit the preparation of microscopic 

 sections is evidently of the same type as the second specimen 

 above mentioned but more completely decomposed and much 



