Hydrous Silicate of Manganese and Calcium. 495 



in druses, and fills veinlets which commonly rest npon or 

 cut the psilomelane and orientite, and is, therefore, later 

 than these minerals. It has been impossible to accurately 

 discriminate between manganite and pyrolusite; the 

 hardness of the crystals corresponds with that of manga- 

 nite, and water was given off when numerous specimens 

 were heated in a closed tube. On the other hand, it is 

 possible to sort out from the material from most of the 

 deposits, a portion that contains considerable crystalline 

 material, which is shown by analysis to contain 90 to 92 

 per cent MnOs, and is therefore undoubtedly pyrolusite.^ 

 Wad is sparingly distributed in the deposits, but appears 

 to be a product of recent weathering. 



Orientite assumes several forms, depending apparently 

 on whether it is abundant or scarce. Where it is abun- 

 dant in the Bueycito region, myriads of small reddish- 

 brown prismatic crystals cover drusy cavities (fig. 1) in 

 psilomelane. Some specimens that appear to be made up 

 largely of psilomelane, are shown by thin sections and 

 polished surfaces to be an intricate mixture of psilome- 

 lane and orientite (fig. 2). In such mixtures, the orientite 

 grains are made up of aggregates of minute tabular 

 crystals. Elsewhere in the neighborhood there are large 

 bodies of tuifs that appear to be impregnated with disse- 

 minated psilomelane and contain 5 to 20 per cent manga- 

 nese. A number of thin sections of such material shows 

 that orientite is universally associated with the manga- 

 nese oxide, but like it forms small grains in the matrix of 

 the tuff, or replaces the glass of the rock fragments, 

 leaving the included feldspars unaltered {fig. 3). Some 

 sections in which the feldspars are quite fresh, show^ 

 diopside crystals which are surrounded by a border of 

 orientite. Such relations suggest that the diopside is 

 more readily replaced than the feldspar. 



Several tliin sections of material from the Bueycito 

 region show orientite pseudomorphs of foraminifera 

 imbedded in manganese oxide (psilomelane f) {fig. 4.). 

 It w^ould appear that this- selective replacement of the 

 foraminifera by the silicate of manganese and calcium is 

 due to the high calcium content of the fossils. 



An amorphous silicate (neotocite f see p. 492) is present 

 in material from Vicente Open Cut No. 15 near Bueycito, 

 from the Abundancia Mine near Manganeso, and the 



"Watson, T. L., Pyrolusite in Virginia, Jour. Wash. Acad. Sci., vol. 8, p. 

 550-560, 1918. 



