0. A. Derby — Manganese Ore Deposits of Brazil. 215 



also show a small amount of rhodonite. A considerable 

 amount of the iron, not separately determined in No. I, is in 

 the form of pyrites, which is tolerably abundant in this rock. 

 The titanium is in part in opaque black grains (ilmenite), in 

 part in transparent red grains with the appearance of rutile, 

 but from which on isolation, I obtained strong characteristic 

 reactions for both titanium and manganese, from which, as in 

 the former paper, the mineral is presumed to be pyrophanite. 



At Piquery it is very evident that the merchantable ore, con- 

 sisting almost exclusively of manganese oxide (psilomelane ?), 

 is clue to the superficial alteration of this rock, involving the 

 subtraction of the carbonic acicl and of a large proportion of 

 the silica, alumina, iron, lime and magnesia. Moreover, it is 

 evident that this alteration has proceeded from above down- 

 ward, or in other words, that it is simply atmospheric weath- 

 ering. So far as observed the ore at this place has lost all 

 trace of the platy (joint) structure of the original rock and, 

 except in the rejected block of low grade ore^all recognizable 

 traces of the original constituents. 



From the neighboring mines of Sao Goncalo and Morro da 

 Mina a type of ore was described that is characterized by a 

 light-colored earthy aspect, platy structure and spongy texture 

 due to innumerable minute rounded cavities. This passes in 

 the upper levels to the ordinary black, massive type of ore 

 (psilomelane ?) in which all these characteristics are lost, and, 

 in one of the lower levels of the Morro da Mina, to a hard 

 black, lustrous type which under the microscope shows minute 

 isolated grains of spessartite in a groundmass of manganese 

 oxide. This groundmass was presumed to be an original con- 

 stituent, and comparable with a specimen of magnetite from a 

 neighboring district in which spessartite occurs in a similar 

 groundmass of iron oxide. The recent observations at Piquery, 

 however, suggest the much more plausible hypothesis that 

 this groundmass is a residue of manganese oxide due to the 

 alteration and leaching of original carbonate and silicate 

 (tephroite) elements like those of the rock at Piquery. The 

 alteration of the latter with retention of the spessartite and of 

 the platy structure is readily conceivable, and this would give 

 precisely the type of low-grade siliceous (from the presence of 

 unaltered spessartite) ore that characterizes the lowest level 

 reached at Morro da Mina. 



Further investigation, which it is hoped can soon be made, 

 will doubtless clear up this and other knotty points in the 

 question of the genesis of this type of manganese ore deposits, 

 which, when of large extent and of commercial value, seem 

 to be due to the alteration of an original rock with predomi- 

 nant carbonate of manganese and tephroite rather than of spes- 



