The dislocations and obstructions which arc found in coal-fields, are known by the names of I. Dikes.
2. Slips.
3. Hitches.
4. Troubles.
The three first relate to the dislocation of the strata, the fourth, or what is termed troubles, relates to the changes which take place in the bed of coal.
A dike is like a wall of extraneous matter, which di videi all the beds of coal found in a coal-field. It varies from a few inches to many fathoms in thickness, and penetrates through the strata to the greatest depth the miner's operations have yet reached ; they are composed of, 1. Sandstone.
2 Greenstone.
3. Basalt.
4. Porphyritic clay mixed with white calcareous spar.
5. Heterogeneous mixture of the coal strata.
6 Water gravel.
7. Quicksand.
8. Old alluvial clay.
9. Recent alluvial clay.
The first five in the order may more properly be named dikes. The four last are more commonly known by the name of gaws, gashes, or chasms.
A slip is a fissure in the strata, from the tenth of an inch to two or three feet in width ; which fissure is filled with rubbish and fragments of all the strata found.
in the coal-field. The strata, on one side, are slipped past those on the °ther side of the fissure, and this dis location receives the name of a slip, where the coal has slipped oft its parallelism lully its own height. These slips arc found from the extent of a few feet, to upwards ol 600 feet perpendicular.
A nitch is a smaller kind of slip, where the disloca tion is from an inch to the extent of the thickness of the coal. In some mining districts, slips and hitches are known by the very appropriate name of steps.
Troubles affect the stratum, or bed of coal, not only as rendering it very difficult to work, but very inferior in quality, and frequently altogether useless as an ar ticle of commerce. These troubles are not found in the chief extent of a coal•field, they only affect it partially, and may be classed under the following heads : 1. Stone in irregular beds.
2. Nips.
3. Shaken coal.
4. Foul coal.
5. Pyritaceous, or brassy coal.
6. Sparry coal.
7. Stoney coal.
8 Black coal.
9. coal.
10. Dike coal ; named, a, Burnt coal.
b. Dander coal.
c, Iluniphed backs.
11. Glazed backs.
12. Troubles in the roof and pavement.
The position and effect of these dislocations and troubles have now to be described.
Dikes, as to their longitudinal direction, arc found not only stretching in the line of bearing through the coal-field, but in every direction, sometimes for many mites nearly in the same straight line, while other dikes havh irregular deflections, but without sharp angular turns. It seldom occurs that great dikes are numerous
in-a district, but the contrary is frequently the case with dikes ol a small size ; they can only be seen in the beds of rivers, or on the sea-shore, but those of and basalt, from their superior hardness, fre quently rise above all the other strata, like a crest or projecting ridge.
Dikes of greenstone, of a few feet to three or four fathoms in thickness, are found sometimes very nume rous, even in a very limited extent of coal field, not exceeding the area of 2u0 acres, lying in various directions, and crossing one another. Fig. 7. Plate CCCLXXXVIII. represents the horizontal section of a coal-field, intersected with greenstone dikes. AB and CD are two dikes lying parallel to e-,ch other, EF and GH are cross or oblique dikes, which not only divide the coal strata, but divide and separate the dikes AB and CD. In looking at a horizontal section of a coal field, the dikes, which intersect it, appear to affect the strata no more than the dividing and separating of them, excepting when greenstone dikes occur, in which case, a change of the coal adjoining them is very evident, as will be noticed afterwards, when the effects of dikes upon the strata in the verticlal section arc treated of. But before entering on this subject, the longitudinal direction of slips will be considered.
Slips run in a longitudinal direction through the strata of a coal-field, and in every obliquity to one another. Fig. 8. Plate CCCLXXXVIII. represents a horizontal section of a coalfield, with two slips, AB and CD, in the line of bearing, which throw the strata down to the crop ; this is the simplest form of slips. Fig. 9. repre sents part of a coal.lield intersected with slips some what similar to a broken sheet of ice. in this Figure, AB is a dike ; all the other lines and ramifications in the figure are slips of every variety and kind, producing dislocations hum the least measurable distaLee to many fathoms; the slips at the points marked a, a, a, disap pear; the lines at C represent four slips of a small size, known by the name of hitches.