23 
character) was only 5ft. Sin. in thickness, and separated from 
the underlying middle coal measures by 12ft. of red clays. 
In the Ocldey Brook, as well as the Norbury section, there 
appeared to be no evidence of a fault, but only the covering 
up of the inferior by the superior strata. 
As a great portion of the future supply of coal must, most 
probably, be looked for under the penman and triassic for- 
mations in Great Britain, it is very essential that all the 
circumstances under which the carboniferous strata disappear 
under those deposits should be carefully ascertained and 
correctly described. When permian or triassic beds are 
found on the rise of the strata they indicate a fault where 
the coal measures have been thrown down, but when they 
are met with on the dip of the strata they may indicate 
a down-thrown fault similar to the one last mentioned, 
or else an overlap of the permian or triassic strata simply 
resting unconformably on the carboniferous strata. Some 
authors have described both these as faults. In the 
beginning of this century certain, geologists and practical 
miners often supposed that when the coal measures disap- 
peared under the permian and triassic strata, generally then 
known as “red ground," they were cut off by a fault, and it 
was useless trying to follow them. “ Bed rock faults ” were 
then used in the same sense whether found on the rise or 
dip of the strata. Now it is of the utmost importance that 
these two classes of phenomena should be carefully dis- 
tinguished, and accordingly most geologists have done so, 
and termed the former a fault because the strata are there 
displaced, and the latter an overlap because the underlying 
strata are not displaced, but simply covered up by the 
superior strata. 
Of course when coal measures disappear on their dip 
under superior beds, they can generally be followed, pro- 
vided there are no faults, and if there are faults the beds 
can be found at some depth or other. Owing to these cir- 
