MANGANESE OXIDES AND MANGANESE NODULES IN MARINE DEPOSITS. 725 



Some of the manganese present as carbonate in the deeper parts of veins may, however, 

 have been derived by emanations directly from the internal metallic nucleus.* 



Dendrites and Coalings of Manganese Dioxide. — The surfaces of rocks containing 

 manganese, on exposure to moist air, become gradually coloured a dirty-brown by 

 the deposition of manganese dioxide, and the internal cracks of many rocks become lined 

 with very beautiful dendrites of the same substance. Even limestones and coral-reef 

 rocks present similar markings where exposed to running water. In the case of coral 

 islands, the manganese which sometimes discolours the coral rock can be traced to the 

 decomposition of the pumice and volcanic minerals, occasionally abundant in the red earths 

 of these islands. 



During the past few years we have examined a large number of the streams flowing 

 into sea lochs connected with the drainage area of the Firth of Clyde in Scotland. On 

 the stones and sandy particles forming the beds of these streams, as well as on the faces 

 of exposed cliffs over which water trickles, abundant deposits and coatings of manganese 

 dioxide were observed. The brilliant shining black lustre of the surfaces of many of the 

 rolled pebbles was due to this oxide. t 



Manganese Dioxide in Suspension in Fresh and Sea Waters. — A considerable 

 quantity of water from various streams flowing into the Clyde Sea-Area was carefully 

 filtered, and on the filters we found traces of manganese in an insoluble condition.| This 

 manganese was present as dioxide, which we believe to have been derived from the 

 mutual attrition of the manganese-coated stones in the beds of the streams, especially 

 during floods. Traces of manganese were also obtained on the filters through which 

 large quantities of sea-water had been passed. This was the case with samples both from 

 the Clyde Sea- Area and from the open Atlantic. This manganese was probably present 

 as dioxide associated with the fine clayey matter which is held in suspension in sea- 

 water, even in some samples collected at great distances from land.§ 



After the water from the Clyde streams was carefully filtered, manganese was found 

 to be present in solution in nearly all the samples in combination with carbonic and 

 humic acids. The manganese was always more abundant in solution towards the head 

 waters of the streams, especially near deposits of peat. 1 1 Its less abundance towards the 



* L. tie Launay, Formation des gttes Me'talliferes, p. 232 ; M. Saralp, Le Manganese des Pyre'nds, (Memoirs de I'Acade'mie 

 des Sciences de Toulouse, 1893). 



+ Stones and sand coated and aggregated with manganese dioxide, and often presenting a polished surface and 

 black metallic lustre, were observed in the Eiver Clyde (Stonebyres Falls, Eutherglen, Underbank) and its tributaries 

 the Mousewater, Hallhill, Diller, Devon, Teiglam, Craignethan, Birkwood, Poniel, Powtrail and Shortcleugh Burns, 

 and Cander and Fence Waters ; also in the streams Mowing directly into the Clyde Sea-Area, as Sea Mill Water, 

 Skelmorlie and Fairlie Burns (Wemyss Bay), and the streams falling into Loch Fyne, Loch Banza, Loch Goil, and 

 Campbelltown Loch. At Middleton Farm, Loch Fyne, there occurs a sandy deposit containing - 7 per cent. Mn0 2 , 

 and at Dundee there is found in alluvium, interlayered with red sand, a black sand containing 2 - 5 to 3 per cent. Mn0 2 . 



t Water was examined in this way from the Clyde, Nethan, Mousewater, Hagshaw Burn, Loch Banza Burn, and 

 Glen Morag Burn. 



§ See Murray and Irvine, "Silica in Modern Seas," Proc. Boy. Soc. Edin., vol. xviii. p. 243, 1891. 



|| The samples in which manganese was found in solution came from Clyde Biver and its tributaries, as, for 

 example, Mouse Water, Hagshaw Burn, Hallhill Burn, Skelmorlie Burn, Glen Morag. 



