The old report also says:
At both quarries the section is similar. The dip is S. 15° E. at 13°, but increases to 22° near a fault, which throws the beds against Carboniferous Limestone. The quarry-face, which is nearly 100 ft. high and is worked in benches, shows the following beds :—
(thicknesses in Ft.)
'Silica Seam' with 'sand' or weathered rock (used for ganister mixture)… 18
Clay (black shale) … 15
'White Seam' good silica-rock with 'spar' (quartz-conglomerate), the 'spar' used for ganister … 20
Black shale (useless) … 3
'Yellow Bed' (best silica) … 16
Black Shale (sold for furnace-seatings) … 4
'Red Bed' (good silica) … 18
The best stone is fine round-grained quartzite. The quartz-conglomerate contains pebbles up to 3 or 4 in. long of vein-quartz set in a quartzose matrix; this is crushed and milled for ganister mixture.
So the sites whose locations are given were open quarries at the time, although there were underground workings that both pre- and post-dated this report.
I suggest the open quarries were the crags that can still be seen immediately above the mines on both sides of the river.