The failure of supplies from Russia during the Russo-Japanese war of 1905 gave fresh impetus to Welsh mines, but this time the greatest contribution came from the neighbourhood of Rhiw, at the western end of the Lleyn Peninsula. In this region, irregular bodies of ore, situated between a massive sill of dolerite below and a lave flow above occur in some mudstones in the Arenig division of the Ordovician System. The mudstone being softer than the associated igneous rock was greatly disturbed – folded and shattered, during a period of mountain building movements, so that the ore – bed was broken into many separate bodies, the content of which varies from as much as 30,000 tons. The thickest masses were about 30 feet thick, having been built up by distortion of a bed originally from 8 to 10 feet thick, but as they were steeply inclined they appeared, in shafts driven vertically through them, to be twice as thick as they really were.

There were two groups of mines, a southern group including Nant Gadwen mine near Llanfaelrhys, and a northern group about a mile away, including the Rhiw and Benallt mines. Ore was being raised as early as 1894, and before the last of them Benallt closed in 1945, they had produced over 195,000 tons of ore. At first, only a few hundred tons a year were raised, but between 1905 and 1907 the output rose to more than 20.000 tons a year. For the next few years most of the ore was obtained from levels driven from an incline at the Nant mine, the most in one year being about 9,300 tons in 1918. The mine has been inactive since 1925. The ore, which was worked from several levels driven off a main incline, came all from one ore body.

The Benallt mine was found to have a particularly complex geological structure, due to folding and faulting, and the ore bodies occurred in a disconnected manner. The character of the original sediments was changed by mineral solutions that invaded the rock as a concomitant of the volcanic activity and the ore consists mainly of manganese silicates with oxides of manganese and iron, is manganese carbonate, so characteristic of the other mines, is present only in significant quantity.

The ore was obtained from opencast workings until, due to the inclination of strata, the amount of over-burden became excessive and inclines were drive into the ore-bearing beds. Eventually several shafts were sunk, one of which was deepened in 1941 from 60 to 130 feet, and the ore was obtained from headings driven from the shaft at three levels, it was allowed to gravitate to the lowest level and from this was hoisted to the surface. Exploratory work shortly before the mine closed in 1945 produced negative results although, having regard to the nature and distribution of the ore bodies, this does not preclude the possibility that they may occur at greater depths.