During the last decade there has been no lack of controversy on the subject of rock classification, but the dispute has concerned igneous rocks almost entirely. Petrography has been threatened by an effort to reduce it to an independent science of igneous rocks, and in consequence of this there has been a tendency to neglect its wider aspects. Important as igneous rocks are, their formation is less well understood than is that of some other sub-groups of rocks ; moreover, they constitute one only of some six or seven sub-groups, and it is the business of petrography as a science to deal with these rocks as a whole. The student of petrography should be encouraged to take this wider view. He shonld look at these different sub-groups of rocks in their proper geological perspective, and approach the study of the classification of igneous rocks with a knowledge of the principles that are applicable to the classification of rocks as a whole.

The discussion concerning the classification of ore deposits has proceeded on independent lines, and in a more well-balanced manner than has that of rocks. With ore deposits, as with rocks generally, genetic principles have triumphed; but the application of these principles to rocks long preceded their application to ore deposits. Iu recent years it has become apparent that the fundamental requirements for the classification of rocks and ore deposits are essentially the same ; and in view of the identity of their interests, both subjects are treated in this paper.

The aims of this paper are : To define the basis of genetic classification ; to give a brief historical account of classification on genetic-geological principles ; to point out defects in the present system of arrangement ; and to suggest an altemmtive scheme of grouping that is in closer accord with geological and genetic principles.

The apparatus to be described has been designed for the cutting of parallel plates and of prisms of crystallized substances in any required direction. The authors have endeavoured to design a small apparatus, simple in construction and easy to manipulate, which at the same time will be capable of yielding results of sufficient accuracy for most optical work on minerals. Several instruments have been devised with the same objects in view, notably those of E. A. Wülfing, G. Halle~ and F. Stöber. The well-known cutting and grinding goniometer of Dr. Tutton is an instrument of an entirely different class.

The mineral hero described was recently collected by the officers of the Geological Survey during their work for the six-inch map of the Island of Mull. It is present as minute phenocrysts in a dark, glassy, magnetite-bearing rock/which is intruded as sheets in the Tertiary lavas nearly one mile south-south-west of Pennygael.

The small rounded crystals of augite are seldom more than ½ mm. in diameter and are traversed by irregular cracks. They contain a few negative crystals, and are occasionally darkened, probably with included magnetite. They appear to have been reabsorbed to some extent by the matrix.

The crystals to which this paper refers are found at Bail Hill, about three miles north of Sanquhar, in Dumfriesshire. At this locality volcanic rocks of Arenlg age outcrop in the midst of the greywacke of the Southern Uplands. The rocks are mainly lavas, and include some coarse-grained tufts which are composed chiefly of augite- and hornblendeandesites. Ill the andesitic fragments, there arc numerous porphyritic crystals of a black augite, which call be easily separated from the rest of the rock. Isolated crystals of augite and hornblende occur also in the matrix of the tufts. No mention of this occurrence is made in Heddle's 'Mineralogy of Scotland', and apparently the only reference to it in previous literature is the short petrographic description in the Geological Survey Memoir.

About the middle of 1911, I was commissioned to report on a copper mine known as the Umkondo, situated in the south-east of Mashonaland, the eastern province of the territory of Southern Rhodesia. The mine is in a very out-of-the-way spot, about 120 miles from Odzi station, the nearest point on the railway, and to reach it took five days of somewhat rough travelling by cart.

The mineral adamite (Zns(AsO4)2.Zn(OH)2), discovered by C. Friedel in 1866 on specimens from Chafiarcillo, Chile, has since been found at only two other localities, namely Cape Garonne in France and Laurion in Greece. At the original locality it would appear to be of rare occurrence, since in the literature mention is made of only two specimens.

It was therefore of interest to find in the British Museum collection a good specimen of adamite from Chafiarcillo. This was found amongst the hornsilvers, and the associated adamite had been thought to be fluorite, which indeed, at first glance, with its violet colour and good cleavage, it closely resembles in appearance. The crystals are remarkable by reason of their very strong pleochroism, and as this feature of adamite has previously received no mention it has been thought worthy of placing on record.