Having stated the effects of dykes, slips, and hitches, as to the of the strata, the changes produced by them upon the coals and strata adjoining have now to be noticed.
When coals approach dykes composed of sand-stone, heterogeneous mixture of stones, water, gravel, quick sand, or el•y, we seldom find any great change in the composition of the coal, excepting that it is either ren dered a little softer or harder, with the natural fissures occasionally very open, and filled with sulphate of lime and pyrites, or with sand or clay when close to dikes of this composition. The strati adjoining are in some cases rendered flatter than in the general coal-field ; in other cases the dip and rise are greatly increased, and the line of level frequently altered. We have, however, to notice a very remarkable and decided change in the stratum or bed of coal, as it approaches dikes composed of grcenstonc, basalt, or porphyritic clay, dikes of greenstone or of basaltic rock, (the common blue whin stone of Scotland,) are much more common in the coal field of Scotland than the porphyry dikes; and their effects are well known, as they are frequently met with in several of the mining districts. The. first indication perceived in approaching one of these dikes, is, that the coal begins to lose its common brightness in the frac ture, and assumes a dull black appearance, but still re tains its common qualities of good burning coal, with very little diffetence; but every foot of advance towards the dike the coal becomes worse, loses every appear ance of common coal and all its qualities; the texture of the coal is lost, and the whole stratum of it becomes a mass of black stoney matter, as before described, when treating of troubles appertaining the bed of coal, under head No. 10. Dike Coal. Adjoining to the dike, the ca vities of this coal are sometimes filled with a soft yellow ish clay, having an unctuous leel ; this clay is also sometimes found in the fissure betwixt the coal and the face of the dike, when the dike is cut through. The coal on the other side is found exactly changed as in the approach to the dike before mentioned, viz. hard, stoney.,
and cellular next the dyke, and these qualities gradually disappearing till the coal assumes its common lustre, with all the qualities of good coal, as found before the deteriorating symptoms appeared on approaching the dike.
When coals approach slips, a variety of changes are found, and they are very seldom met without symptoms appearing of their approach, "particularly if they are of considerable magnitude. The most common symptoms of their approach are, numerous hitches, swellings in the roof, sudden risings or wavings of the pavement, the coal much stronger in its texture than common, and more difficult to work ; at other times much softer than usual, and this softness at times increases to such a de gree as to lose all firmness of texture. In this state it is sotnetimes like coal dust mixed with small cubical pieces of good coal ; at other times it is found uncom monly triable, somewhat like soot. The occurrence of iron pyrites is common, in some instances filling fissures and cavities, having a beautiful golden lustre, and crystallized, some of the crystals forming cubes of more than half an inch in size, and finely laminated ; in other instances, the pyrites are so run into and en twisted with the coal as to form a very heavy mass of singular texture. We also find the sulphate of lime very abundant near to slips, filling up the numerous fis sures of the coal, and enttvisted with it coals of the soft and of the sooty quality above mentioned; where the pyrites and sulphate of lime abound, the coats are of no use as fuel. An increase of water from the fissures, as a dike or slip is approached, is also common; and when the slip is found, very great feeders of water sometimes occur. If the coal is dry, inflammable air (if found ge nerally in the coal field) comes off in great quantity ; and there are instances of .inflammable air being found at a dike or slip, though never seen before in the district.
Sand-dikes, or gashes containing quick sand, are dan gerous from the quantity of water they contain, which is found in some instances so great as to drown and destroy the collie' y, if not prevented' by powerful means.