J. Clifton Ward— On Rock Staining. 391 



more or less, merely by the action of the atmosphere, but such changes 

 would probably be much helped forward and increased by the infil- 

 tration from above of carbonated water, together with the hydrate 

 and carbonate of iron. Hence it should not surprise us that a 

 porous, readily-weathered grit should assume the purple colour in 

 places far removed from limestone agency ; though the fact, that the 

 purple colour is most common where there has decidedly been a lime- 

 stone overlap, seems to support the view here maintained. 



Mr. Lucas has mentioned a number of sections in which the lime- 

 stone rests on unstained rocks ; but such are evidently exceptions, 

 when it is considered that for several miles the limestone immediately 

 overlaps red rocks, and that these had in the first place been mapped 

 as possibly belonging to the Permian, until further evidence showed 

 how clearly they were of Millstone -grit and Coal-measure age. I 

 have alluded to the non-colouring of beds in places as due to the 

 following reasons : ^ " 1st. Changes in mineral composition of the 

 rocks to be coloured : thus some grits contain much red felspar, 

 others but little ; some' sandstones and shales contain more mica and 

 more iron oxides diffused through them than others. 2nd. Changes 

 in composition of the overlying limestone. 3rd. Differences in the 

 porosity of the underlying rocks, some withstanding the infiltration 

 of the carbonated water more than others. With regard to this 

 third point, one would naturally expect, as is the case, that permeable 

 grits would be coloured to a greater thickness than impermeable 

 shales ; while such a rock as the Calliard, mentioned as occurring 

 near Barwick-in-Elmet, being so flinty and close-grained, might 

 resist the percolation altogether, and accordingly it is found to be 

 quite uncoloured, although its joints are lined witli carbonate of 

 lime. The upper parts of the coloured shales would likewise become 

 marly by reason of the calcareous matter deposited within them." 



Mr. Lucas makes the strange statement that the Magnesian Lime- 

 stone " does not colour the fields red, when free from drift;" but 

 surely the drift in those parts takes its nature very largely from the 

 rocks beneath : there is little or no red drift-clay on Coal-measure or 

 Millstone-grit ground near Leeds, while it is found at once upon the 

 Magnesian Limestone area; and Prof. Eamsay says,^ "The Magnesian 

 Limestone soil is always red." ^ 



In conclusion, Mr. Lucas states (p. 343), ''"Were Mr. Ward's ex- 

 planation the true one, the lower limestone ought certainly, at least 

 here and there, to be reddish." But since the iron is chiefly in the 

 form of hydrate (2Fe203*3H20), it could not be expected to impart a red 

 colour, except where, exposed to the atmosphere, it becomes anhy- 

 drous red peroxide (FeaOg) and forms a red soil; or when, as 

 carbonate (FeCO^), soluble in excess of carbonic acid, it is carried 

 away from the limestone and redeposited as red peroxide, the carbonic 

 acid being liberated. The black oxide (FeO'FejOa), Prof. Sedgwick 

 says, is also constantly associated with the yellow limestone. 



^ Q,uart. Journ. Geol. Soc, vol. xxv., p. 296. 

 2 Quart. Journ. Geol. Soc, vol. xxvii., p. 242. 



^ On Mr. Aveline's working copies over the Lower Magnesian Limestone, I find 

 many notes of " red soil " and " red clay." 



