- 22 DR JOHN MURRAY AND MR ROBERT IRVINE ON THE 



belonging to extinct species, dozens of ear-bones and other bones of cetaceans, myriads 

 of small zeolitic crystals, cosmic spherules, and numerous fragments of highly altered 

 volcanic rocks were brought up with the manganese nodules, and these bodies not 

 unfrequently formed the nuclei around which the manganese was deposited. These 

 manganese nodules, and the other interesting substances associated with them, are de- 

 scribed and figured with considerable detail in the " Challenger " Report on Deep- 

 Sea Deposits.* 



In the present paper we propose to point out the distribution of the oxides of man- 

 ganese in the geological series of rocks, in fresh and sea water, and in marine deposits, with 

 special reference to our explorations in the lochs of the west of Scotland ; to give an 

 account of investigations undertaken to ascertain the source of the manganese present in 

 marine deposits in the form of the higher oxides, and thereafter to discuss the various 

 views that have been advanced to explain the formation and distribution of manganese 

 concretions in marine deposits in general. 



Manganese in Eruptive and Schisto-Crystalline Rocks. — Manganese is present in 

 nearly all the crystalline rocks, generally only in such minute traces that it does 

 not usually appear in analyses of these rocks owing to its not having been specially 

 looked for. It increases in amount along with the iron, and is more abundant in 

 the basic than in the acid series of rocks.t In some fragments of basic volcanic 

 glass from the bed of the Pacific, Murray and Renard found 0'34 and 0"44 per 

 cent, of manganous oxide (MnO) present as silicates. | In a large number of rocks 

 which we examined, the quantity of manganous oxide (MnO) ranged betw r een 

 - 01 and 1*0 per cent. In the unaltered crystalline and schisto-crystalline rocks 

 the manganese exists as protoxide in combination with silicic acid. When these 

 rocks had undergone alteration, some of the manganese was present as carbonate. § 



* See Murray and Eenard, Deep-Sea Deposits Chall. Exp., London, 1891. 



t The following are some of the manganese-bearing silicates (the numbers after the names indicate the percentage 

 of MnO) :— Paulite, 0-6 ; diallage, 5-20 ; augite, 0-3 ; acmite, 1-3 ; rhodonite, 54 ; hermanite, 47 ; segerine, horn- 

 blende ; pyrosinalitc, 21 ; astropbyllite, 10 ; tephroite, 70 ; knabellite, 35 ; zephrolite ; manganese-alumina garnet ; 

 pyrochlore, 7 (somewhat variable) ; tantalite, 1-G. 



X Murray and Rkxard, op cit, p. 307. 



$ I. In the following rocks, chiefly from the Clyde drainage area, the manganese oxide was soluble in carbonic 

 and dilute acetic acids, therefore presumably present as carbonate : — 



Felstone, from 



Blackhill, 







contained from 



)! 



j> 



Devonside, 









>J 



)) 



>! 



Gourock (2 samples), 





)> 



)! 



>> 



Innerleithen 



> 







H 



I) 



)! 



Lanark, 









)) 



)) 



)> 



Lesmahagow 



(9 sampl 



es), 



» 



„ (Green) 



)> 



)! 









)) 



Breccia (calcareous), 



» 



Glenfalloch, 









)) 



Limestone, 



» 



Kilsyth, 









>> 



Mud (25 fathoms), 



>> 



( 'umbrae, 









)1 



Sandstone (new red), 



» 



Hamilton, 









)) 



Tuff (volcanic), 



»» 



Eaglesham, 









» 





Percent. MnO 



0-5 to 1-0 





trace 





0-1 to 0-5 





0-1 to 0-5 





0-5 to 1-0 





0-5 to 1-0 





0-1 to 0-5 





0-5 to 1-0 





0-5 to I/O 





0-1 to 0-5 





0-5 to 1-0 





0-1 to 0-5 



