Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/392

 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 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 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 magnesian limestone from Garforth, east of Leeds, to Knaresborough, striking across partly coal-measures and partly millstone-grit, there is no such thing as "Lower Red 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 overlying 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 coloration 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 Red 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