1868.] MAW VAEIEGATED STRATA. 381 



sedimentary green layers and spots to which the bleached slate is 

 adjacent may have received as an accession the sesquioxide of 

 iron abstracted from the surrounding zone. 



The case of discoloured blotches from which the iron has been 

 discharged, without any centre of aggregation, is more difficult to 

 account for ; but some examples of banded slates in the Penrhyn 

 Quarries, given in fig. 31 (PI. XIY.), seem to suggest an explana- 

 tion ; the light discoloured bands are concentrically surrounded by 

 a dark band, darker than the slate, apparently by an excess of the 

 colouring oxide, as though the oxide of iron had been dispersed 

 centrifugaUy to the outline instead of aggregated to the centre ; and 

 it was observed that the mechanical layer forming the centre of the 

 discoloured band is lighter in colour than where no such external 

 dark zone occurs. 



The other form of secondary variegation, viz. the conversion of 

 the blue and purple slates to green in large fields of colour, and to 

 green slate in contact with the intrusive greenstone dykes, appears 

 to have no relation to that just described. An analysis (No. 29, by 

 Dr. Voelcker) of an example from the Penrhyn Quarries showed 

 that it contained 8-26 per cent, of iron, equivalent to 10-63 per 

 cent, of protoxide as compared with (Analysis No. 27) 11*40 per 

 cent, of sesquioxide in the purple slate of the same quarry. There 

 is therefore no material departure of colouring oxide from the green 

 variety, and we must look for an alteration in its state of combina- 

 tion to account for its change of colour. 



The slates of a uniform green colour have no stratigraphical hori- 

 zon distinct from that of the blue and purple varieties. The highest 

 beds of slate in the Penrhyn Quarries (fig. 29. PI. XIV.), underlying 

 the uppermost Cambrian grits of Bronllwyd, and marked separately 

 10 and 11 in Professor Pamsay's section, Ko. 59 (p. 156, Geology 

 of North Wales, Memoirs of Geological Survey), exhibit, on a large 

 scale, this interchangeable colouring. The highest portion of JSTo. 11 

 is more generally green, and purple prevails in No. 10 ; but there is 

 no distinct line of demarcation between the two colours, which inter- 

 lace with each other in a direction vertical to the stratification. 



The purple (Analysis, No. 27) contained 



Protoxide of iron 0-874 



Sesquioxide of iron 6'540 



Sulphur 0-031 



The green (Analysis, No. 28) contained 



Protoxide of iron 5-49 



Bisulphide of iron 0-15 



A second analysis of No. 28 gave 5*90 per cent, of protoxide of 

 iron. The change of colour is therefore due to the conversion of 

 the sesquioxide of iron into protoxide ; there was also about one- 

 sixth less iron in the green than in the purple, a difference insuffi- 

 cient to explain the change of colour, but accounted for by the fact 



