1868.] MAW VARIEGATED STRATA. 355 



Lime 012 



Magnesia 0'U4 



Potash OUl 



Soda 0-44 



Water with traces of organic matter . . . 5-85 



99-89 



Tho following analyses, however, indicate that the depth of colour 

 of similar strata charged with the red anhydrous sesquioxide is less 

 directly related to the amount of colouring-matter present than to its 

 state of subdivision ; for instance, the red clays of the Argile plastique 

 of tho Paris Easin, and of the variegated Neocomian beds near Beau- 

 vais, contain nearly 20 per cent, of red sesquioxide, whilst many 

 sandstones of similar colour contain less than from one-fourth to 

 one-eighth of the amount. In the one case it is in a state of line 

 subdivision, evenly disseminated^ throughout, and intimately asso- 

 ciated with the mass, whilst in the red sandstones it frequently 

 occurs merely as a surface-coating to the individual grains, as, for 

 example, in the red Millstone-grit of Cumberland, and part of the 

 Keuper of Shropshire. 



In the Old Red Sandstone of Forfarshire the colour seems to be 

 evenly disseminated through the grains, and to have been derived 

 from the breaking-up of an older red rock. In tracing the sources 

 of materials from which red beds have been derived, the mechanical 

 condition of the colouring oxide is a point of considerable interest, 

 as indicating whether they were formed from original red beds or by 

 the association of ferruginous matter with the other detritus at the 

 time of deposition. 



Many grey and bluish-grey beds contain a large proportion of 

 sesquioxide of iron. Its precise condition seems to be scarcely un- 

 derstood, and to be worth fuller investigation. In some cases its 

 normal red or yellow colour is evidently obscured by the presence 

 of carbonaceous matter ; but in many grey beds which contain, by 

 analysis, a large proportion of sesquioxide of iron, the amount of 

 carbon is not nearly enough^to account for the obliteration of the nor- 

 mal red or yellow colour. The colours of the anhydrous and hy- 

 drous sesquioxide are given in figs. 1 & 2, PI. XI. Burnt red 

 earthenware, red bricks, and Yenetian-red may be cited as familiar 

 examples of the colour of the anhydrous form. The hydrous ses- 

 quioxide imparts a tint ranging from dull brown to bright yellow ; 

 and its presence in association with the anhydrous form tends to 

 reduce the brilliancy of the colour of red beds. There are also se- 

 veral lower hydrates of sesquioxide intermediate in colour between 

 the fully hydrous and the anhydrous form ; but it is not practicable 

 to distinguish them by analyses in the presence of aluminous and 

 other hydrates. 



When occurring separately in juxtaposition, as in the Folliestone 

 beds of the Lower Greensand and in the Lower Bagshot beds, the rich 

 blood-red of the anhydrous sesquioxide and the golden yellow of 

 the fully hydrous sesquioxide form a marked contrast ; but when 

 they are concentrated to the extent of 15 or 20 per cent, in the matrix, 



