BKOADfJATE. — The " Red Rocks '' and Associated Beds. 83 



iron, the two analyses are almost identical. The total content of iron 

 does not differ by more than 04 per cent., but, while in the green 

 argillite the proportions of ferrous and ferric oxide are 3:1, in the red 

 argillite the proportions are equal. This shows that the difference in 

 colour is due to the presence of more ferric iron in the red argillite, and 

 indicates that the change of colour has been brought about by the pro- 

 duction of ferric iron from the ferrous iron present in the green argillite. 



In discussing the formation of red sediments it seems a common premise 

 that the iron-content in the sediment was in the higher state of oxidation 

 at the time of its deposition, either as a hydrated sescjuioxide, as supposed 

 by Joseph Barrell (23, p. 286), or already in the form of anhydrous red 

 haematite, in which state I. C. Russell concludes all red sediments were 

 deposited (24, p. 56). 



J. D. Dana has attributed the red colour of certain shales to the oxida- 

 tion of their iron-content by the action of heat resulting from orogenic 

 movements (25). How the oxidation has been brought about is not 

 stated. Some such hypothesis as Dana's seems best suited to the case of 

 \h.Q red argillites of Wellington Peninsula. 



The connection between the red argillites and the strike-faults of the 

 Maitai rocks is indicated by the map. That the areas of red argillites have 

 suffered from faulting-eft'ects is shown by the fact that a quartz lode is 

 developed in connection with each band. As stated above, these lodes 

 are silicified fault or shear zones (26, p. 135). Genetically, however, they 

 are segregated veins, as distinct from true fissure-veins, and as such have 

 been described by J. Park (27, p. 64). Siliceous solutions, circulating 

 mostly in a downward or lateral direction, may be considered efficient 

 agents in supplying the oxygen necessary to convert the ferrous iron to 

 the ferric state. 



In the field the appearance of the green argillites is consistent with the 

 idea of leaching ; the argillites are of a dull greyish-green colour, and, 

 although quite compact as distinct from weather-rotted, they are without 

 the sheen commonly noticed in light-coloured argillites (11, p. 47 ; 29, p. 50). 



The effects of vein solutions on country rock composed of " clay slates, 

 greywacke slate, and similar rocks " have been investigated by A. von 

 Groddeck (28). He finds that the result of such action on " variegated 

 slates " will be a leaching-out of iron and magnesia, a loss in sericite, and a 

 gain in quartz. The final product, however, is a slate composed of '' quartz 

 and sericite with a little rutile and considerable specular iron." 



On comparing the analysis of a typical dark-coloured argillite with 

 that of the red argillite it will be seen that the small differences in the 

 analyses vary in accordance with von Groddeck's results ; while the per- 

 centage of silica shows a slight increase in the red argillite, the amounts 

 of alumina and potash are slightly less. The total loss on ignition is high 

 in the case of the dark-coloured argillite, due presumably to the carbon- 

 aceous matter present. In the case of the red argillite, water-content is 

 probably responsible for the ignition-loss of 3'42 per cent. 



The leached-out products of iron and magnesia are not necessarily lost 

 to the rock ; in the case of the slates investigated by A. von Groddeck the 

 resulting product has " considerable specular iron." Probably the leached 

 iron, as in the case of the Wellington argillites, has been oxidized and 

 redeposited in the rock. 



Discussing the origin of red formations, Joseph Barrell (23, p. 290) con- 

 cludes that the chief factors operating in the production of red shales from 



