109 



the coal, under favourable circumstances, easily breaks down in 

 parallel opipedal masses. 



Faults. 



In Chapter VII., on Dislocations, &c, our observations were 

 principally confined to horizontal and diagonal movements ; but 

 in sedimentary or more or less horizontal beds, these kinds of 

 disturbances are not so much felt as the vertical displacements 

 called " faults." They are first produced by the splits and trans- 

 verse fractures of the primary base, as already explained. The 

 north and south lines are called main faults, and the east and 

 west cross faults. When the strata rest on a semifluid base, 

 such as beds of clay and moist carbonate of lime, the divisional 

 planes disturb the uniformity of the pressure, consequently the 

 isolated masses will gradually subside or elevate according to the 

 angle in which they are separated. The most simple and fre- 

 quent case of faults is represented in Plate XXI. Let us consider 

 what would be the consequence of the oblique separations of 

 the series of beds in Fig. 1, supposing they rested on a soft 

 stratum. According to the laws of statics, the pressure will in- 

 crease as the lower area of the masses becomes less than the 

 upper, and vice versa : the weight divided by the area will give 

 the amount of each, therefore the wedges will sink and the parts 

 between them rise to restore the equilibrium of the pressure on 

 the semifluid base, as illustrated in Fig. 2. The planes or joints, 

 when they are found in contact, are often seen grooved and po- 

 lished by the friction or the rubbing of one side against the 

 other : and as the direction of the action must necessarily de- 

 pend on local variations, the polished striae are not always pa- 

 rallel, but commonly curved and irregular. Sometimes the beds 

 are separated vertically upwards of 1000 feet, yet the masses 

 of the uplifted and depressed strata do not present such irregu- 

 larity on the surface ; the action having been so extremely slow, 

 that the upper parts have been modified and obliterated by at- 

 mospheric and aqueous causes ; during the displacement there- 

 fore w T e only see the wreck of the elevated masses in the shape 

 of sand, clay, &c, and the greater part of this washed away. 



It has been supposed that even these " faults " were the result 

 of violent mechanical convulsions produced by volcanoes, but 



