362 ' PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



red and yellow interlace, here and there accompanied by nodidar 

 concretions of segregated sesquioxide. 



Another phase of variegation, noticed in Mr. Pengelly's memoir 

 on the red beds of Devon, consists of discoloured zones concentrically 

 surrounding a pebble, fragment of fossil, or other mechanical nucleus, 

 and evidently localized by the occurrence of such nucleus. The Old 

 Bed beds of Scotland (fig. 14, Plate XII.) and the red conglomerates 

 of Devon (fig. 5, Plate XI.) contain frequent examples of such loss 

 of colour ; and fig. 4 (Plate XL) represents zones of discoloration sur- 

 rounding mechanical fragments of shale in the Gres bigarre, near 

 Eaon I'Etape, Yosges. 



Among the various mechanical nuclei that seem to have deter- 

 mined the position of these spheres of discoloration, fragments of 

 Devonian limestone in the red beds of Devon are of frequent occur- 

 rence. In a discussion on a previous paper relating to this subject, 

 read during the late session of the Society (1867), it was suggested 

 by Mr. Godwin- Austen that the fragments of carbonate of lime 

 might have arrested the peroxidation of the iron, or reduced the 

 sesquioxide to a state of protoxide in immediate contact with them, 

 and thus have produced the zone of discoloration without removing 

 the metallic base. 



I have obtained through the kindness of Mr. Pengelly an example 

 of such discoloration from Torbay (fig. 5, Plate XI.), and procured 

 a determination of the iron and hme in the red ground and the area 

 of discoloration. 



Analysis ^o. 9, of the discoloured zone, indicated 0-81 per cent, of 

 iron almost wholly in a state of sesquioxide, with a small amount, 

 viz. 0-15 per cent., of protoxide. 



Analysis N'o. 10, of the red ground, gave 2-88 per cent, of iron, 

 nearly all of which occurred as sesquioxide, and about 0-10 per cent, 

 as protoxide. 



Both the red and the discoloured portions contained the same 

 amount of lime, viz. 7*84 per cent. No connexion can therefore be 

 traced between the chemical action of the limestone and the bleach- 

 ing of the adjacent zone ; and we have here again merely the depar- 

 ture of the greater part (in round numbers three-fourths) of the 

 colouriQg oxide, without any alteration in the state of its combi- 

 nation : indeed it seems apparent that the action cannot be merely, 

 if at all, a chemical one ; for fragments of Trap and other rocks pro- 

 duce the same effect in the Devonshire red beds, and in the Old Eed 

 beds of Porfarshire (fig. 14, Plate XII.) and the red beds of the Gres 

 des Yosges and of the Gres bigarre of the Yosges district (fig. 4, Plate 

 XL) fragments of various shales and rocks are concentricaUy sur- 

 rounded by similar zones of bleaching. 



The great majority of examples of variegation in which there are 

 neither segregated nor mechanical nuclei exhibit similar conditions, 

 viz. differences in the light and dark parts merely as regards the 

 proportion of iron present, without any alteration in its state of com- 

 bination, or change in the composition of the matrix. 



The following analyses are of red Bunter sandstone, mottled 



