386 proceedings: geoi^ogicaIv society 



upper Eocene age, such as foraminiferal limestone, shaly, glauconitic 

 sandstone, conglomerate, and waterlaid andesitic tuff, and in igneous 

 rocks such as latite-porphyry and latite. Eocene time was charac- 

 terized in the area of Oriente Province chiefly by subsidence, with 

 active volcanoes, causing interbedding of volcanic and sedimentary 

 rock. 



The ore consists of some or all of the oxides pyrolusite, psilomelane, 

 manganite, and wad, and braunite has been noted, but as a rule the 

 individual minerals are not readily distinguished. The deposits are 

 of three general types according to their associations : deposits in bedded 

 rocks, deposits in irregular siliceous masses (jasper, bayate) that occupy 

 openings in both sedimentary and igneous rock, and deposits of nodules 

 and fragments in clay. The "bedded" deposits comprise several varie- 

 ties, one of the most common having been formed by partial replace- 

 ment by manganese oxides of portions of tuff beds, and consisting of 

 poorly consolidated, tuffaceous material, granules of pink clay, zeolites, 

 and manganese oxides. Other bedded deposits are replacements of 

 limestone, sandstone, and conglomerate, and a fossil bog was noted. 



The proximtity of volcanic rocks to the manganese-bearing areas, 

 and the broader structural relations, suggest the possibility that the 

 manganese was derived from volcanic rocks of the Sierra Maestra 

 Mountains, transported by artesian waters, together with silica and 

 deposited in the porous tuff strata and as masses of manganiferous 

 jasper in joints and fissm-es in the limestone and glauconitic sand- 

 stone. The jasper and bedded tuff, in weathering, have contributed 

 the manganese that is now found in the other rocks and in detrital 

 deposits. 



D. F. Hewett: Summary. — Most manganese deposits offer two 

 rather distinct problems: (i) that which is concerned with the fea- 

 tures, source, manner of deposition, and distribution of certain primary 

 minerals, only a few of which are useful in the arts, and (2) that which 

 is concerned with the features, source, manner of deposition, and dis- 

 tribution of certain secondary oxide minerals which are derived from 

 the primary minerals and are highly useful. 



Recent work shows, although the common primary manganese 

 minerals, such as carbonates, silicates, and sulphides are widely found 

 in deposits that are associated with igneous intrusions, there are a 

 number of poorly defined manganiferous carbonates and silicates 

 that are laid down with sediments. In some places these minerals 

 form concretions in the sediments but elsewhere they appear to be 

 uniformly distributed in thin zones. 



Although the higher oxides and hydrous oxides of manganese appear 

 to be deposited in the belt of weathering only, the lower oxides are 

 apparently deposited under conditions found below the belt of weath- 

 ering, or may be formed when the higher oxides are affected by regional 

 metamorphism . 



