Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/542

376 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 9, material, — by infiltration doubtless ; but whence derived, is a problem yet, I think, to be solved.

The beds have thus undergone change — have been doubly metamorphosed, — first in the introduction of the iron, and secondly in the altered form in which the iron is now exhibited in the cellular ironstone.

There can be little doubt that at some period the iron, subject to local variations as to proportion, was equably diffused throughout the mass of impregnated material (even as now it is in the slightly ferruginous brown sandstones of the Northampton beds) : it afterwards, in obedience to some subtle principle yet to be determined, separated from the associated material, and arranged itself as walls of cells, ever varying in form (some being rectangular, many irregular and many-sided and often with rounded angles, some nearly spherical, and occasionally concentric), but all containing cores of the original material — whether sand, marl, or clay — from the majority of which the iron has almost entirely departed.

The quantity of iron present in these beds offers also a fertile subject for consideration. The ore yields on an average 40 per cent, of pig-iron — sometimes more than 55 per cent. From this Duston pit alone, more than 1000 tons of ore per week are sent away ; and the weekly produce from the county of Northampton is from 9000 to 10,000 tons, yielding from 3000 to 4000 tons of pig-iron. This has been going on for some ten or twelve years, certainly not always at the same rate as now ; but were we to dot out on the map the comparatively few excavations from which already such an immense amount of iron has been obtained (say a million and a half of tons), and compare the area of these with the whole area occupied by the ferruginous beds of the county, we should arrive at such an idea of the aggregate quantity of iron imported into these beds subsequently to their deposition, as to involve in considerable difficulty the question of its derivation and of the conditions under which it was introduced.

South of Duston is the east and west branch of the valley of the Nen, and immediately south of Northampton is the conflux of this branch with the northern branch of the same valley. About two miles west of this point commence thick alluvial beds, which occupy, with a bordering of Upper Lias Clay, the Nen valley and its tributaries for many miles to the east and north-east. The upper bed here consists of an earthy clay with much vegetable matter ; and in its lower portion it is spotted bright blue by nodules of Vivianite or phosphate of iron : it contains at its base numerous remains of ox, deer, horse, and wild boar, frequently stained blue by the same mineral. The lower bed consists of a sandy gravel, from which I have obtained teeth of Elephas antiquus and primigenius, bones of Hippopotamus, and teeth both of the upper and lower jaws of Rhinoceros tichorhinus. This alluvium overlies the Upper Lias clay.

On the southern side of the valley is Huntsbury Hill, a projecting