1868.] MAW VARIEGATED STRATA. 357 



Analysis No. G4, by Dr. Yoelcker, Kimmeridgc Clay, Chapman's 

 Pool, near Kimmeridgc, Dorsetshire. 



Protoxide of iron 1-45 per cent. 



Sesquioxide of iron 1-04 ,, 



Bisulphide of iron 086 (f^-^'f- ^^ 



SiJphate of lime 0*35 per cent. 



Carbonate of lime 34-28 „ 



Analysis No. 65, by Dr. Yoelcker, Kimmeridge Clay, near Calne. 



Protoxide of iron 2-08 per cent. 



Sesquioxide of iron 4*32 , , 



Bisulphide of iron I'^sji^of ."I'o-ee 



Sulphate of lime 5*34 per cent. 



Carbonate of lime 428 ,, 



ALQalysis No. 66, by Dr. Yoelcker, Oxford Clay, Brick-works, Canal 

 Bank, Chippenham. 



Protoxide of iron 1'12 per cent. 



Sesquioxide of iron 3'25 ,, 



Bisulphide of iron I'l^ {fr'^of !!" oll 



Sulphate of lime 1*37 per cent. 



Carbonate of hme none. 



The carbonate of protoxide has but a weak colouring-power ; and 

 the grey colour of the strata in whch it occurs is generally due to 

 the presence of carbonaceous matter, which also obscures the colour 

 of any accompanying sesquioxide of iron. 



Bisulphide of iron occurring in mechanical admixture is common 

 to beds of all ages and colours, and has but little colouring-power. 



Iron pervading the older slate rocks occurs in almost every state 

 of combination, and in all varieties of proportion, in some of the 

 Cambrian slates almost exclusively as sesquioxide, and in others as 

 protoxide. 



Iron also occurs to some extent, with a weak colouring-power, as 

 the basic sulphate of sesquioxide in beds in the lower part of the 

 Ashdown Sands. Its production from the decomposition of iron 

 pyrites will have to be referred to in accounting for some forms of 

 variegation. 



3. 0)1 the Primary Condition of Iron in Red Beds. — As a neces- 

 sary introduction to the subject of the blotching of ferruginous 

 strata, reference must be made to the question discussed by Dr. 

 Dawson, on the origin of the colour of red beds, and the primary 

 condition under which the iron was deposited. Dr. Dawson (Quar- 

 terly Journal of the Geological Society, vol. v. p. 2b), in describing 

 the red beds of Nova Scotia, suggested that the sesquioxide of iron 

 pervading them was derived from the oxidation of bisulphide of iron 

 under the influence of heat and moisture ; he stated that bisulphide 

 of ii'on is largely developed in the older rocks of Nova Scotia, and 

 that there is no apparent earlier source for the iron in a state of 

 sesquioxide. For the details of Dr. Dawson's views I would refer 



