The labour of tracing the history of the Red Silver Ores, of collating the literature, and of tabulating the crystalline forms previously observed, has been, to a large extent, rendered unnecessary by the memoir published by E. Rethwisch (Neues Jahrbuch für Mineraloqie, Beilage Band iv. (1886), pp. 31-109), under the title "Beiträge zur mineralogischen und chemischen Kenntniss des Rothgültigerzes."

Mynydd Mawr is a conspicuous hill with a craggy face towards the north, and a well-marked corrie (Cwm Du) on that side. It lies to the south-east of Moel Tryfan and the west of Llyn Cwellyn (Carnarvonshire). On the Geological Survey map it is represented as a somewhat rounded oblong patch of felstone, about 1⅜ mile from east to west, and a mile, or a little more, from north to south, intrusive in Lower Silurian (Ordovieian) rocks, which are probably Arenig. I passed some years since to the north of the above-named face, and collected specimens from erratics which were plentifully strewn over the lower land. On microscopic examination I found them to contain a very puzzling and remarkable mineral which I have long intended to describe, but kept deferring this in hope of being able to revisit the place to obtain more specimens for examination, and by taking these from rock in situ, to prevent any question arising as to the true source of the rock.

Specimens of this rock were presented a short time since to the British Museum by the Earl of St. Germans, and Mr. T. Davies, knowing my interest in pierites and peridotites, kindly requested me to examine them and communicate the results to the Mineralogical Society. The rock occurs, as I am informed, in boulders between Menheniot and St. Germans. The picrite of the Clicker Tor, near the former place, is well known and has been more than once described, but, as all specimens which I have seen from that locality are very different, macroscopically and microscopically, from those now sent to me, it may be well to put on record a description of the latter, which, after the excellent summary of the mineral character of picrites given in Mr. Teall's British Petrography, need not be lengthy.

Some of the published analyses of specimens of Dufrenite have afforded results not in good accord with the chemical formula 2Fe2O3, P2O5, 3H2O, usually ascribed to that species.

Moreover, since analyses of a crystallised Dufrenite from Cornwall gave results showing a higher ratio of acid to base than demanded by the above formula, a re-examination of the ordinary botryoidal variety of the mineral seemed desirable.

This mineral is found as a constituent of a variety of gabbro occurring on the shore at the village of Coverak, Cornwall. The rock is an aggregate of allotriomorphic grains of chrome-diopside, labradorite and olivine. The olivine is, in general, perfectly fresh, but it is traversed by a network of veins in which magnetite has been deposited. The whole of the iron-ore present in the rock occurs in these veins in the olivine ; so that in this, as in other respects, the rock bears a close resemblance to some of the gabbros described by Prof. Judd from the Western Isles of Scotland.