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The Forest of Dean by H. G. Nicholls

Occasionally this conglomerate


Probably a twentieth part of the above total should he added to the amount charged, in consideration of the quantity consumed by the colliery engines, thus making the gross annual produce a third of a million of tons.

CHAPTER XVI.

_The Geology of the Forest, and its Minerals_--Their character in general--Description of the beds of conglomerate, mountain limestone, iron veins, millstone grit, and lower coal measures--"The Coleford High Delf"--Elevation of the Forest range of hills--The middle coal veins--The upper veins--Mr. Mushet's analysis of the Forest coal--Their fossils--The stone-quarries of the district.

The geological conditions of the Forest of Dean merit careful observation, not only as regards the mineral wealth comprised within its limits, but as explanatory of its undulations, and the means of maintenance for its inhabitants.

The strata of the Forest repose in a basin-like form, the greatest depression being near the centre; the longer axis extending from N. to S. about eleven miles, and the transverse axis, in the widest part, ranging from E. to W. about seven miles. The general observer, if he takes his stand on the edge of hills by which this basin is bounded, will see the enclosing character of the ridge, as well as the less conspicuous circle of somewhat elevated land occupying the central

portion of the field, and which is separated by a valley or plain from the surrounding ridge.

This outlying ridge marks in most places the outcrop of the Conglomerate, Mountain Limestone, Iron Veins, Millstone Grit, and Lower Coal-measures.

Mr. Maclauchlan's geological map of the district exhibits the course of the conglomerate bed, and the consequent disappearance of the old red sandstone formation under the Dean Forest basin. Occasionally this conglomerate, or hard grit, forms two distinct beds, very distant from one another, near Lydney for instance, and on the Kimin Hill and Buckstone, although it is sometimes cut off altogether by a "fault," as opposite Blackney. It varies in hardness as well as in the number of the pebbles, and not unfrequently presents an abrupt fall at its termination, as at "the Harkening Rock" in the Highmeadow Woods.

[Picture: General view of the centre of the Forest]

[Picture: Geological Map of the Forest]

The upper portion of the bed is soft, and acquires the character of the limestone clay, often throwing out springs, such as St. Anthony's Well, which have accumulated in the limestone rocks above. A very micaceous stone sometimes occurs in the upper parts, having the appearance of silver: hence the name of "Silver Stone" given to a spot near the Hawthorns, where it is found. The surface which the carboniferous limestone exposes is also represented in the map. The Forest coal-field is surrounded by this formation, with the exception of the line of fault between Lydney Park and Danby Lodge, a distance of four miles.

The principal iron-mine train of the district divides into a lower or more crystalline, and an upper or more argillaceous and sandy stratum. Mr. Mushet thus describes this important metallic vein:--"The iron ores of the Forest of Dean, which have become intimately known to me, are found, like the ores of Cumberland and Lancashire, in churns or caverns formed in the upper beds of the mountain or carboniferous limestone. The leaner ores contain a great deal of calcareous matter in the shape of common limestone or spar, which reduces the percentage in the ore as low as between 15 and 25 per cent., and it seldom exceeds 25, except when mixed with fragments of what is called brush ore, which, when in quantity, raises the percentage to 40 or 45. Brush ore is a hydrate with protoxide of iron, and frequently, if not much mixed with calcareous earth, contains from 60 to 65 per cent. of iron. These ores are found in chambers, the walls of which are exceedingly hard limestone, crystallized in rhombs. This limestone is called the 'crease,' and is frequently found enveloped and covered with the iron ore. The miner has to cut his way through this crystallized limestone from chamber to chamber, a distance of from 20 to 100 yards, before he reaches the next of these deposits, which are sometimes found to contain 3,000 or 4,000 tons of ore. The principal part of the ore is then dug easily, somewhat like gravel; but the sides of the chambers are often covered with the stony ore before described, which requires gunpowder to detach it from the rock." These various ores were found by the same excellent authority to yield iron in the following proportions:--


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