24 O. A. Derby — Manganese Ore Deposits 



of psendomorphs of microscopic garnets. Another locality, to 

 be discussed below, proves that this interpretation is correct and 

 that the pitting of the rock is due to the disappearance of 

 included garnets. Other specimens from the same opening, in 

 which the structure above described is obscured by secondary 

 manganese oxide, give a residue with some sound garnet, with 

 a minute quantity of ilmenite and, on ignition with nitre, a 

 tolerably abundant reaction for graphite. 



The Morro da Mina, situated some 6 to 8 kilometers to the 

 northeastward of the town, is a high hill covered quite uni- 

 formly with outcropping, or loose, ore and thus presenting the 

 most extensive ore deposit yet known in the district. As, 

 however, this ore is more siliceous than the present high 

 requirements of the market admits, very little has been done in 

 the way of development except in the loose superficial material 

 that gives little insight into the structure of the ore body ? but 

 fortunately an old abandoned mining tunnel, apparently driven 

 by some deluded gold prospector, gives a good section of a 

 considerable portion of the mass and below the zone of surface 

 action. As at Piquiry and Sao Groncalo, granite occurs in the 

 immediate vicinity of the hill but none was seen in close con- 

 tact with the ore body. Prospecting operations in the neigh- 

 borhood show that the ore continues for a considerable distance 

 both northward and southward from the hill, and there are 

 strong indications that in the latter direction a more or less 

 continuous line of outcrops connect this locality with that of 

 Agua Limpa, some ten or dozen kijometers distant. 



The above mentioned mining tunnel, which unfortunately 

 could only be examined by the insufficient lighting of matches, 

 extends for about 25 meters in very hard somewhat sheared 

 manganese ore and terminates in soft unsheared clay that evi- 

 dently represents some massive rock decomposed in situ. An 

 assay sample taken by an experienced prospector at every two 

 meters shows that the mass is tolerably uniform in structure 

 and composition and that, except in comparatively insignificant 

 patches, the rock is perfectly fresh. This sample is understood 

 to have given on analysis about 40 per cent of metallic man- 

 ganese, thus corresponding very closely with the analysis given 

 below of a picked specimen, if, as is presumable, the metallic 

 contents was only determined in the soluble portion, the 

 abundant residue being set down as quartz without further 

 examination. This ore body has the appearance of a vertically 

 sheared dike, or intercalated bed of which one side is free, 

 forming the steep slope of the hill, while on the other side 

 comes the above mentioned clay mass that separates it from 

 another smaller and parallel ore body that outcrops on the top 

 of the hill at a distance of some dozens of meters. 



