nEVAN — ON THE SOUTH WALES COAL-FIELD. 
98 
Nowbridge, in Glamorganshire, othei-wise called Pontypridd, is 
situated on tho anticlinal axis, which has tho effect of bringing up 
the lower measures at Macsteg iron-works in the Llynvi valley. It 
is marked by the appearance of a rock termed the " Cockshort 
Rock," which consists of a bed, or beds, of sand, consolidated 
together so as to form a quartz-rock : by miners it is known in 
some districts as " carreg," or " craig gwenith faen" — tho wheat- 
grained stone, and is important as being the only white quartz-rock 
in the field ; it is a serviceable guide to the relative position of 
certain beds of coal. Its course is from Baglan, near Briton Ferry, 
to Cwm Avon, Maesteg, Braich-y-cymmer, across the Llangeinor 
Mountain, Cillely, and Newbridge, to the Eglwysilan Mountain. We 
find the trough again, although wonderfully narrowed, at the 
western end of the field, at Llanelly, Caermarthenshire. The smaller 
trough is directly south of the larger one, and in fact occupies aU the 
distance between the great anticlinal axis and the southern crop of 
the basin. As a consequence, the beds of the south crop are much 
more highly inclined than those on the north. They are also of 
much greater thickness, showing a progressive tendency to thin-out 
as they approach their northern limit. 
The faults of the South Wales field are numerous, and often locally 
1 extensive, though there are none of general magnitude like the 
! Ninety Fathom Dyke. The largest faults are to be found in the 
i north-eastern part of Glamorganshire, running south-west from 
Merthyr, across the Gellygaer-hill to Llancaiach, where it is one 
hundred yards, so that the Mynyddswlyn-vein of coal, which is 
worked by level at Tophill, is obHged to be worked by deep pit at the 
Llancaiach Colliery, only a few hundred yards to the south. 
Another great fault, runs westward from Trevethin, near Pontypool, 
to Blackwood, where there is a perfect chaos of faults, the appearance 
which on the Geological Survey's map reminds one strongly of 
Bradshaw's Railway-map. The whole of the south crop is much in- 
tersected by faults, particularly in the west of Glamorganshire and 
Caermarthenshire ; but their small size prevents their having any 
general interest. Nevertheless, it is highly important that even 
small faults should be duly chronicled, for many instances might be 
recorded where ignorance of existing disturbances has caused a 
