296 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 12, 



sandstone," that '^red oxide of iron, less generally diffused than 

 the preceding, forms the greater part of the colouring-matter of the 

 lower gypseous marls, also of many of the red beds in other parts 

 of the series," that black oxide exists " in the form of black spots 

 generally stellated and of dendritic impressions, rarely forms the 

 colouring-matter of considerable masses," and that iron-pyrites is 

 rare, but " probably exists in a state of minute subdivision in some 

 earthy beds which are liable to effloresce." Probably the coloration 

 of the underlying beds is due to both the peroxidation of iron already 

 existing in the sandstones and shales, and to the introduction from 

 above of colouring-matter, the former cause being very likely the 

 most potent. 



The non-colouring of the underlying beds in places is probably 

 due to several 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 

 the 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 thick- 

 ness 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 accord- 

 ingly it is found to be quite uncoloured, although its joints are lined 

 with carbonate of lime. The upper parts of the coloured shales 

 would likewise become marly by reason of the calcareous matter 

 deposited within them. 



I have thus been led to conclude that along the base of the mag- 

 nesian limestone from Garforth, east of Leeds, to Knaresborough, 

 striking across partly coal-measures and partly millstone-grit, there 

 is no such thing as "Lower Eed Sandstone" or " Rothliegende," 

 but that any red or purple shales, sandstones, or grits occurring along 

 this line are simply coal-measure or millstone-grit beds, as the case 

 may be, which have been coloured through the agency of the over- 

 lying limestone. 



Mr. Aveline has mentioned to me that the Coniston grits as they 

 approach the carboniferous-limestone country of Ulverstone, where 

 haematite is so abundant, are stained red, such a colour being nowhere 

 else seen along their line of strike — but that in other parts, where the 

 limestone, free from iron-ore, is not far removed from the grits, no co- 

 loration is observable. The clay-slates, however, do not exhibit any 

 coloration when approaching the limestone haematite districts ; this 

 may be because they are less permeable. Again, Mr. Aveline tells 

 me that the Silurians beneath the Old Eed Sandstone near Kendal 

 are also stained red. 



"Whether the red marls and sandstones beneath the magnesian 

 limestone, all along the eastern edge of the Yorkshire coal-field, will 

 turn out to be but coloured coal-measure beds, I leave others to 



