386 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



Carbonate oj lime 35-58 



Magnesia 0-91 



Alkalies and loss 0*35 



Alumina 2'44 



Matter (exclusive of the iron) insoluble in hydro- 

 chloric acid 52*51 



10000 



The principal difference in the composition of the red and ochreons 

 portions consists in the presence of a large proportion of carbonate of 

 lime in the latter, which has evidently been infiltrated from the 

 overlying calcareous band, accompanied by a change to the colour 

 prevailing in the more calcareous parts of the "Wenlock shale. In 

 this case, however, there is no increase in the proportion of pro- 

 toxide to sesquioxide of iron. The yellow colour of the Magnesian 

 Limestone lying in the midst of red beds, may perhaps be an ana- 

 logous case ; and the entire absence of red beds from the calcareous 

 Oolites must also be noticed ; it is clearly independent of the amount 

 of iron present, as nearly all the grey and yellow beds of the Oolites 

 (see analyses Nos. 64, 65, ^^, p. 357) contain similar proportions of 

 iron to those in the various red beds. 



Although an increase, from extraneous sources, of the calcareous 

 element in any red bed seems to induce bleaching and discoloration, 

 it does not bear a perfectly regular ratio to the proportion of lime 

 present, as discoloration has taken place in some red beds con- 

 nected with the infiltration of a less amount of carbonate of lime 

 than that originally present in others that have not been so changed, 

 and a blood-red colour pervades the base of the chalk at Hun- 

 stanton. It may, however, be generally stated that a bright-red 

 colour is a character of non-calcareous strata, and dun- colour, or 

 grey, of calcareous beds, quite irrespectively of the amount of iron 

 present. 



I will not attempt any explanation of these facts, and merely 

 offer them as worth further investigation, and requiring a larger 

 series of observations before any reliable conclusions can be sug- 

 gested. 



11. On the Condition of the Iron in the Depleted Areas of Red and 

 Purple beds. — Before leaving this part of the subject special refer- 

 ence must be made to the circumstances attending the colour and 

 state of combination of the iron in the buff and green blotches of 

 slates and red beds. It is important to bear in mind that their 

 colour bears no definite relation to the proportion of the iron which 

 they contain. As a rule the blotches contain a much smaller pro- 

 portion of iron than the red, purple, or blue ground on which they 

 are disposed ; but whilst a certain proportion of iron may form the 

 colouiing base of one red bed, it may contain actually less than the 

 light blotches on another ; and, furthermore, the great bulk of the iron 

 may in each case be in a state of sesquioxide ; so that the mere abs- 

 traction of a part of the iron will not by itself explain the local 

 reduction of the red colour to a paler hue. 



The discoloured blotches vary in tint from a light clear sea-green 



