1868.] 



:MA\V VAHIEWATED STRATA. 



;v.)n 



In the last stage (c) the eojicentrie arrangement disappears, and the 

 great bulk of the copper is accurauhitcd as a central nucleus in the 

 condition of rich copper regains. In large lumps several such nuclei 

 may be formed. 



The various chemical changes taking place in the several stages, 

 Avhich have been investigated by Liirzer, are fully described by 

 Br. Percy ; but a satisfactory explanation of the character of the 

 motion of the copper towards a central nucleus seems wanting. Jt 

 cannot be the result of the mechanical aggregation of fused particles, 

 as the heat is not carried nearly to the melting-point, or even to 

 plastic fusion ; indeed the action is arrested beyond a certain tempe- 

 rature, and concentration is not the only phenomenon. 



Mr. D. Forbes states that when silver is present in the ores, it 

 appears to travel outwards, and that he has seen some specimens in 

 which the outer surface of the piece of roasted ore was covered by a 

 thin shell of metallic silver, as if electro-deposited. The phenomena, 

 if not analogous, bear a curious resemblance to the kind of changes 

 which have taken place in the Northamptonshire iron-ore beds, in 

 which the protoxide of iron, and phosphoric and carbonic acids, have 

 been aggregated towards definite centres, whilst the sesquioxide has 

 been repelled from such centres in concentric ferruginous bands, 

 travelling outwards until arrested by mutual contact. 



I believe that many of the phenomena connected with banded 

 agatescent and other concretions will be found analogous in cha- 

 racter to these phenomena, and to the ferruginous banding of yellow 

 strata — their accumulation having taken place in convergent or retro- 

 gressive lines within a solid matrix, producing a structure resembling 

 the mechanical superposition of successive coats. 



14. Disposition of Manganese in Variegated Strata. — Another case 

 of secondary variegation, resembling, though independent of, the 

 occurrence of iron, which 



may here be noticed, is Fig. 64. — Carboniferous Grit and Sand- 

 represented in fig. 64, of stone, Willey Park, Shropshire, con- 

 sandstone and grit occurring taining Black Oxide of Manganese. 

 at the base of the Shropshire 

 Coal-measures. The black 

 fields of colour are due to 

 sesquioxide of manganese : 

 they vertically intersect suc- 

 cessive beds of conglomerate 

 and sandstone ; and their 

 disposition is obviously due 

 to some cause independent 

 of mechanical arrangement, 

 analogous to the forces that 

 have operated in the rear- 

 rangemcnt of iron. 



15. General conclusions. — 

 In comparing the composi- 

 tion of the different coloured areas of variegated strata, one of the most 



VOL, XXIV. PAUT I. 2 V 



