390 /. CUfto7i Ward—On Reck Staining. 



near Barwick-in-Elmet, immediately beneatli the limestone, are 

 sandy, micaceous, and purplish, the purple colour being quite un- 

 known to me among the Lower Coal-measures, although I have 

 mapped them almost continuously from Sheffield to Leeds. Further 

 south, sandstones and shales belonging to the Middle and Upper 

 Coal-measures are in places similarly stained beneath the limestone 

 escarpment. The Plompton grit is felspathic and micaceous ; gener- 

 ally, between Knaresborough and Collingham, of a purple tint, and 

 readily crumbling through the action of the atmosphere. Where it 

 has an east and west strike, and passes away westwards from the 

 limestone, it loses the purple or red colour. Mr. Lucas has stated 

 (p. 342) that " only one and the same bed is found to be purple," 

 whereas, besides the Coal-measure shales described above, some of 

 the beds beneath the Plompton grit exhibit a tendency to like coloura- 

 tion. To quote from my paper,^ " Below this (Plompton-grit) come 

 some 50 feet or less of such sha,les as generally occur in the Millstone- 

 grit series, followed by a sandstone, gritty in some parts, and generally 

 of a reddish colour, though not so markedly coloured as the last- 

 mentioned grit; this rock, which may average some 75 feet in thick- 

 ness, passes down into sandy and flaggy shales, in many parts having 

 a tendency to be both gritty and of a purplish tint." 



These general facts led me to infer that either the colouring-matter 

 contained in the grit and shale beds was unusually brought out, — that 

 is, that the iron in them was more than ordinarily peroxidized, — by 

 some local cause, or that colouring- matter had been introduced by 

 some local cause. Now the only cause I know of is the neighbour- 

 hood of the Magnesian Limestone, and its having formerly over- 

 lapped these very beds. The opinion that this was the main cause 

 seemed, moreover, to be strengthened by the fact that generally the 

 purple colour was most developed along a line immediately beneath 

 the limestone, though in some cases the colour is quite absent even 

 there. I have stated it as nay belief that the colouration " is chiefl}^ 

 due to the peroxidation of iron ; and this, it seems to me, may take 

 place in two ways : either by the action of carbonated water from 

 the limestone above filtering through porous grits and sandstones, 

 and converting the protoxides contained in them into sesquioxides ; 

 or by iron being brought from the overlying limestone, in the form, 

 of hydrate and carbonate, and redeposited in the rocks below." 

 Prof. Eamsay considers the last explanation the most probable- 

 Prof. Sedgwick says,^ " Hydrate of iron appears to form the colour- 

 ing-matter of many of the yellow beds of limestone." Prof. 

 Phillips believes * the purple colour to be due to decomposed ferru- 

 ginous mica, and the absence or presence of mica in the diflerent 

 parts of the same beds of grit or shale might thus account for the 

 absence or presence of colouration. This decomposition of the mica, 

 or other chemical changes occurring in the rocks, might take place, 



^ Quart. Journ. Geol. Soc, vol. xxv., p. 293. 



2 " On the Red Eocks of England of older date than the Trias." Quart. Journ. 

 Geol. Soc., vol. xxvii., p. 242. 



* Transactions of the Geol. Soc, 2nd series, vol. iii,, p. 239. 



* "Notes on the Geology of Harrogate." Quart. Journ. Geol. Soc, vol, xxi., p. 234. 



