Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/391

 sandstones generally supposed to be Permian, occurring along the eastern edge of the Yorkshire coal-field immediately below the magnesian limestone.

The first of these two points in favour of a Permian age can go for little, inasmuch as grits and conglomerates of all ages are likely to resemble one another in structure. The second point, however, deserves more consideration, and mainly for two reasons : —

1st. That the purple tint is unusual among millstone -grit beds proper.

2nd. That this is the tint generally assumed by the so-called " Lower-Red-Sandstone " beds immediately beneath the magnesian limestone.

Red grits, as stated before, are not unfrequent in the millstone - grit series ; thus, in a section of a bore -hole made on Bradford Moor, the rough rock is termed "Red Sandstone," and in another case " Old Red Sandstone ; " while in several parts of the Yorkshire millstone-grit area between the Lancashire and Yorkshire coal-fields, various members of the grit have in places a red colour. The purple tint, however, is rarely, if ever, met with in large masses. In one spot only, far removed from the limestone, have I ever observed it ; this was in a shaly band occurring among Third-Grit sandstones, three miles north of Leeds, and five west of the limestone escarpment, on the turnpike road between High Moor Allerton and Hill Top, the nearest limestone outlier being distant three miles.

On the other hand, the purple colour of the Plumpton grit is by no means constant ; thus a little north-east of Spofforth, on the road to North Deighton, the magnesian limestone in Newsome-Bridge Quarry is seen lying upon a very uneven surface of whitish grit with no trace of red or purple coloration ; while but one-eighth of a mile distant, in St. Helen's Quarry, the grit is coarse and purple, and has lying upon its denuded surface a bed of red marl, at the thickest part five feet, and overlapped towards either end of the quarry by yellow limestone ; the marl contains in one part a thin layer of apparently redeposited grit. I would here also observe that the lower part of the limestone, wherever it rests on grit, is apt very frequently to contain fragments of the latter or scattered quartz-pebbles.

As the result of my observations in general, I should infer that the frequent red and especially purple colour of beds immediately underlying the magnesian limestone is due in some way or other to the limestone covering, and its former extension further to the west. The coloration, I should presume, is chiefly 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. Sedgwick says, in the same article before quoted, that " hydrate of iron appears to form the colouring-matter of many of the yellow beds of limestone, also of many of the beds of marl and