366 PKOCEEDINGS OF THE GEOLOGICAL SOCIETT. [April 22, 



There is somewhat more alumina in the red than in the bleached 

 part, and a much larger proportion soluble in hydrochloric acid; 

 but this appears to be due merely to local variations in the composi- 

 tion of the clay, and to be unconnected with its variegation ; otherwise 

 the general composition of the red and bleached parts is similar, the 

 main difference being in the small amount of sesquioxide in the buff 

 (Analysis 'No. 47) compared with that in the red (Analysis l^o. 46) — 

 the amount of protoxide in the two colours being nearly identical, 

 but slightly in excess in the buff portions. The change seems difficult 

 to connect with infiltration from above, as isolated blotches of the 

 buff occur entirely environed by the body of the red clay. 



5. On Discoloration and Bleaching connected tvith Joints. — Variega- 

 tion, which has evidently been more or less locahzed by the exist- 

 ence and position of cracks and joints, seems to be of several distinct 

 kinds ; one of which may be described as the simple rusting of the 

 joint-surfaces in beds containing carbonate of protoxide of iron, and 

 is evidently connected with surface-infiltration, for it always dimi- 

 nishes in proportion to the depth and distance from the surface. 

 Fig. 20 (Plate XIII.) represents an example of grey marl that has 

 become thus variegated ; and fig. 22 (Plate XIII.) represents some 

 beds of the Upper Purbeck marble, near Corfe, Dorsetshire. The 

 contrast of colour between the blue centre of the slabs and the 

 ochreous crust following the Kne of joints is very marked, but it is 

 met with in some degree in aU jointed beds containing protoxide of 

 iron. This form of secondary variegation is, weU explained by the 

 following analyses, by Messrs. Johnson and Sons — 



No. 17. Hue Purbeck marble (fig. 22, Plate XIII.) contained 



Protoxide of iron 4-681 -»yr .ni- • ^ a.aq 



c • -J r- •, .on }■ Metaiiic iron 4*4o ; 



Sesquioxide oi iron rAJ j 



No. 18. The rusty crust of No. 17, adjacent to joint, contained 



Protoxide of iron 2-88 1 tv*- ^ „n • • ^„ A.on 



c, • • J r • o on f Metallic iron 4:'zU, — 



Sesquioxide oi iron 2*80 J 



and is evidently due to the partial peroxidation of the carbonate of pro- 

 toxide, there being but little difference in the amount of iron present. 

 This example is particularly instructive, as bearing on the theory of 

 the alleged dissolution of the colouring oxide to account for the 

 bleaching of red beds : here is an extreme case of a bed charged 

 with the least insoluble oxide of iron, under circumstances most favour- 

 able to its removal by dissolution ; but instead of its solution, the 

 iron along the joints has become converted into the comparatively 

 insoluble sesquioxide, unaccompanied by any material diminution in 

 its amount. Fig. 19 (Plate XIII.) represents a somewhat similar case 

 of joint-rusting in the Great OoKte of Northamptonshire; but in 

 this example the rusting not only follows the joints, but concentri- 

 cally surrounds some isolated balls of clay ; and it was probably in- 

 duced by their hygroscopic property. 



In contrast vfith the rusting of joints, the other form of joint- 

 variegation must be noticed, viz. the bleaching of rocks that are 

 coloured by the red sesquioxide along the lines of jointing. The 



