The lignite in this region also warms the ranchman's cabin, being easily mined where a seam is exposed in the walls of a ravine or on the side of a hill.
Rich strips of alluvial soil, however, seam a cold clay-marl, needing intensive cultivation to become highly productive.
In the oldest form of this class of working, where the size of the pillar is equal to the width of the stall or excavation, about 4 of the whole seam will be removed, the remainder being left in the pillars.
To secure the perpendicularity of the shaft, it is necessary to leave a large mass or pillar of the seam untouched around the pit bottom.
With careful packing it is estimated that the surface subsidence will not exceed 40% of the thickness of the seam removed, and will usually be considerably less.
In the top seam, faded but legible, was written Annie in very small print.
The dark bituminous layers of clay slate, which occur intercalated among the quartzites, have led, here as elsewhere, to the hope of coming upon a seam of coal, but it is contrary to experience that coal of any value should be found in rocks of that age.
The deep, short gorges and glens which seam the southern slopes of the Caucasus are inhabited by Ossetes, Tushes, Pshays and Khevsurs in the west, and by various tribes of Lesghians in the east.
A multitude of ravines and gullies, filled with torrential streams or dry, according to the season of the year, and characterized by many beautiful cascades, seam the narrow coastal plain and the flanks of the mountains.
The bed of fire-clay under a coal seam, being impervious to water, frequently determines the horizon of numerous springs issuing from the hillsides.
The name is from a word meaning "to roast till puckered" or "drawn up," in reference, it is suggested, to a peculiar seam in their mocassins, though other explanations have been proposed.
Number and nature of the coal seams in new ground, or the position of the particular seam or seams which it is proposed to work in extensions of known coalfields.
I bore is commenced at the dip, and reaches a seam of coal A, at 40 fathoms; at this depth it is considered proper to remove nearer to the outcrop so that lower strata may be bored into at a less depth, and a second bore is commenced.
I was 40 fathoms in depth, we multiply the depth by the rate of inclination, 40 X 8 = 320 fathoms, which gives the point at which the coal seam A should reach the surface.
This derangement being carefully noted, another bore to the outcrop on the same principle is put down for the purpose of proving the seam C; the nature of the strata at first is found to agree with the latter part of that bored through in No.
3, but immediately on crossing the dislocation seen in the figure it is changed and the deeper seam D is found.
The first process in laying out the workings consists in driving a gallery on the level along the course of the coal seam, which is known as a " dip head level," and a lower parallel one, in which the water collects, known as a " lodgment level."
A coal seam with a soft pavement and a hard roof is the most subject to a " creep."
It will be seen that by this method the whole of the seam, with the exception of the pillars left to protect the main roadways, is removed.
At Monceaux les Mines, in France, a seam 40 ft.
In 1873 there could be seen, in the thick coal seams of Bengal, near Raniganj, a seam about 50 ft.
In the South Wales system of working, cross headings are driven from the main roads obliquely across the rise to get a sufficiently easy gradient for horse roads, and from these the stalls are opened out with a narrow entrance, in order to leave support on either side of the road, but afterwards widening to as great a breadth as the seam will allow, leaving pillars of a minimum thickness.
The main level or gate road is driven in the benches coal, or lower part of the seam, while a smaller drift for ventilation, called an air heading, is carried above it in one of the upper beds called the slipper coal.
In driving levels it is necessary to cut grooves vertically parallel to the walls, a process known as shearing; but the most important operation is that known as holing or kirving, which consists in cutting a notch or groove in the floor of the seam to a depth of about 3 ft., measured back from the face, so as to leave the overhanging part unsupported, which then either falls of its own accord within a few hours, or is brought down either by driving wedges along the top, or by blasting.
Cattle sometimes congregate in cold weather around a burning coal seam and enjoy the warmth.
The average thickness of the seams worked is from 12 to 18 ft., but occasionally a seam attains a great thickness - 20 to 80 ft.
Numerous smaller channels seam the whole face of the country carrying off the surplus drainage in the rains, but drying up in the hot season.
The "Thick Seam" of Margherita, in Upper Assam, averages 50 ft., and in some places reaches as much as 80 ft.
But there is generally a certain depth of alluvial cover which requires to be deducted, and which we call a fathoms, then (40 - 3 =37)X 8 = 296 fathoms; or say 286 fathoms is the distance that the second bore should be placed to the rise of the first, so as to have, for certain, the seam of coal A in clear connexion with the seam of coal B.
The carrying frame, while the work is going on, is fixed in position by jackscrews bearing against the roof of the seam, which, when the cut is completed, are withdrawn, and the machine shifted laterally through a distance equal to the breadth of the cut and fixed in position again.
At Vierfontein a seam of coal is worked above it.