FAULTS 



329 



intersected with greenstone dykes. A B and c D are two dykes standing parallel to 

 each other ; K F and o H are cross or oblique dykes, which divide both the coal-strata 

 and the primary dykes A B and c D. 



2. Slips run in straight lines through coal-measures, and at every angle of incidence 



847 

 crop. 



dip. 



to each other. Fig. 847 represents a ground plan of a coal-field, with two slips A 'B 

 and c D, the line of bearing of the planes of the strata, which throw them down to 

 the outcrop. This is the simplest form of a slip. Fig. 848, exhibits part of a coal- 

 field intersected with slips, like a cracked sheet of ice. Here A B is a dyke ; while the 

 narrow lines show faults of every kind, producing dislocations varying in amount of 

 slip from a few to a great many fathoms. The faults at the points a, a, a vanish ; and 

 the lines at c denote four small partial slips called hitches. 



The effects of slips and faults whether produced by dykes, or otherwise on the 

 coal-strata appear more prominently when viewed in a vertical section, than in a ground 

 plan, where they seem to be merely walls,. veins, or lines of demarcation. Fig. 849 is 



rise 



a vertical section of a coal-field, from dip to rise, showing three strata of coal a, b, c. 

 A B represents a dyke at right angles to the plane of the coal-beds. This rectangular wall 

 merely separates the coal-measures, without affecting their line of rise ; but further to 

 the rise, the oblique dyke c D interrupts the coals a, b, c, and not only disjoins them, but 

 has produced a movement which has thrown them and their concomitant strata greatly 

 lower down ; but still, with this depression, the strata retain their parallelism and 

 general slope. Nearer to the outcrop, another dyke, E F, interrupts the coals a, b, c, 

 not merely breaking the continuity of the planes, but throwing them moderately up, 

 so as to produce a steeper inclination, as shown in the figure. It sometimes happens 

 that the coals in the compartment H, betwixt the dykes c and E, may lie nearly hori- 

 zontal, and the effect of the dyke E, F, is then to throw out the coals altogether, leaving 

 no vestige of them in the compartment K. 



The effect of slips on the strata is also represented in the vertical section,/^. 850, 

 where a, b, c are coals _. ft 



with their associated 

 strata, A B is an inter- 

 secting slip, which throws 

 all the coals of the first 

 compartment much lower, 

 as is observable in the 

 second, No. 2 ; and from 

 the amount of the slip, 

 it brings in other coal- 

 seams, marked 1, 2, 3, 

 not in the compartment 

 No. 1. c D, is a slip pro- 

 ducing a similar result, but not of the same magnitude ; B F represents a slip across 

 the strata, reverse in direction to the former ; the effect of which is to throw up the- 



