FROM THE LOWER CARBONIFEROUS OF NORTH WALES, 271 
stained limestone, rather impure, rests on green and purple shales, 
with a sharp line of demarcation, but without signs of erosion or 
unconformity. Occasionally the lower beds of the limestone are 
interstratified with shales so as to form a passage*, There is 
always a perfect conformity in dip. 
Distribution and Thickness—The accompanying section (p. 269) is 
drawn to a true scale of 6 inches to the mile, the horizontal distances 
and heights taken from the Ordnance 25-inch maps, supplemented by 
aneroid measurements. The dip of the Bastard Limestone in the 
dingle is 28°; this diminishes towards the top of the dingle to 
12°. No dip is then to be got till we reach the Carboniferous 
Limestone, which dips at 9° to 10°. From these data we get an 
apparent thickness of a thousand feet for the conglomerates. It is 
probable that this far exceeds the reality, through the inequality of 
the surface of Wenlock shale and through the possibility of a roll in 
the limestone. Making allowance for this, their thickness cannot 
be much less than 500 feet. At Penlwys (Pile House), where we 
have been able to make a direct measurement in the hill-side, we 
find the thickness to be 130 feet, and west of Colwyn Bay they have 
thinned out to nothing, the rate of attenuation being about | in 34. 
EKastwards the conglomerates thin out equally rapidly, and finally 
are overlapped by the Carboniferous Limestone at a distance of three 
miles from Ffernant. The conglomerates become finer as the beds 
become thinner, and tend to pass into shales and sandstones, afford- 
ing evidence that the thinning and thickening of the beds is due to 
their original unequal distribution, rather than to their partial 
removal by denudation before the deposition of the Carboniferous 
Limestone. 
The position and appearance of the beds in Ffernant Dingle favour 
the idea of their having been deposited against a bank or sloping 
surface of Wenlock shale, probably one side of a broad hollow cut 
out by denudation in the surface of these rocks. The irregular 
distribution. of the conglomerates is probably due to such inequa- 
lities in the surface on which they were deposited. 
It may be mentioned that a fine section is exposed in the valley 
of the Clwydog, near Ruthin, showing Carboniferous Limestone rest- 
ing conformably on red micaceous shales with thin sandy beds, and 
thrown against similar beds by a fault ranging north and south 
through Berth. In the bed of the river at Berth brecciated and 
conglomeratic limestone, dipping N.E. at 5°, forms the base of the 
deposit ; it rests on Wenlock shale dipping 8.W. at 30°. Conglo- 
meratic beds occur in the series in the side of the highroad to 
Ruthin, half a mile west of Llantwrog. The pebbles are similar to 
those of Ffernant. 
Origin of the Pebbles—In the Bastard Limestone there occur a 
few fragments of Wenlock shale, all angular and evidently of local 
origin. With the exception of these, the pebbles are all waterworn 
and generally completely rounded. A few of the less fissile and 
* Mr. G. H. Morton states that beds of limestone and conglomerate are 
interstratified near Denbigh. 
