84 SEVENTH REPORT—1837. 
before coming to a master fault, to meet with one, two, or more parallel 
smaller ones, throwing the measures up or down in the same direc- 
tion. 
The master faults appear in general to run across the whole basin, 
and to extend into the old red sandstone. 
Minor faults occasionally branch from the larger ones, and perhaps 
in some instances two very considerable faults merge into one. But 
it appears when such is the case both the faults throw the measures the 
same way. 
The dip of the measures is often different on the opposite sides of a 
fault ; hence it happens that running along the course of the fault the 
down-throw or up-throw necessarily increases or diminishes. 
There are instances of faults which, while they are considerable in 
the middle of their course, diminish to nothing at both extremes. 
The author observes, “I have been informed that there are faults 
which, while extensive at the outcrop of the measures where the beds 
dip with the greatest rapidity, diminish towards the centre of the basin, 
the beds on the opposite sides then crossing one another and reversing 
their relative positions; but Iam not able to point out any instance. 
Faults of this description might arise from horizontal movements, and 
there are symptoms of such movements in the limestone between 
Pwll Du Bay and the Mumbles Point.” 
The faults are seldom quite perpendicular, and it appears that in 
general their dip or underlie is towards the down-throw. Hence it 
would happen that when a block of strata lies between two up-throw 
faults, it would have the form of a wedge with the point downwards, 
and the two faults would vertically merge into one. No working in 
the Welsh coal basin has yet been deep enough to get to the bottom of 
any of these wedges. To this general direction of the underlie there 
are, however, many exceptions, particularly in the east and west faults 
occurring between the Turch and Tawe rivers, in the neighbourhood 
of the limestone irregularly thrown up in Cribbarth Mountain, and 
producing what are technically called leaves by the Welsh miner, a 
leaf being nothing more than the duplication or over-lapping of a bed, 
occasioned by a fault dipping at a very acute angle in respect to the 
horizontal plane towards the up-throw side. 
The faults are of various breadths, and it would be natural to sup- 
pose that those which produce the greatest step in the measures 
should be the widest. But there are instances where master faults 
are not more than a few inches wide; and others, where faults that 
occasion a step of only a few feet, are said to be a hundred yards or — 
upwards in breadth. But the author thinks the dimensions of these 
very wide faults are often exaggerated, as coal miners are accustomed 
to state the width of a fault to be the distance from good, solid, profit- 
able ground on the one side to the same on the other, while probably 
the disturbed part may include several small faults. 
Another circumstance connected with these faults is very import- 
ant. The coal on one side of a fault is frequently very different in 
quality, as respects the quantity of bitumen it may contain, from that 
