So far as I am aware no account of the microscopic structure of this rare and beautiful rock has yet been published. I have, therefore, thought that a brief description of its structure and some considerations concerning the origin of tourmaline rocks may be of interest to petrologists.

Boulders of the rock, as is well known, are abundant in the vicinity of the village of Luxullian (about five miles from the town of St. Austell, Cornwall), where I collected specimens in tho autumn of 1873; but I believe the rock itself has never been discovered in situ, though veins of different varieties of tourmaline rock are abundant in the granite of this and other districts in Cornwall, some of which occasionally rather resemble it, and the mineral frequently occurs in the granite itself, the felsite cleans, and the altered sedimentary rocks.

The Geological Survey Map of CornwalI shews, to the south of the town of St. Austell, several patches of “greenstone,” extending from near the ancient earthwork called Trethullan Castle, on the west, to the sea at St. Austell Bay in the east ; a distance of over four miles.

The western end of the most extensive of these patches swells ont on the top of the hill to the north-east of St. Mewan Church, so as to cover several hundred acres (including some of the best land in the parish). This was described by Mr. J. Arthur Phillips some years since in the Philosophical Magazine. Mr. Phillips speaks of the rock as being distinctly crystalline, consisting of felspar (sometimes triclinic) ; semi-transparent yellowish-brown crystals, probably hornblende ; a green fibrous mineral also believed to be a variety of hornblende ; many black grains of oxide of iron and a few hexagonal crystals, probably apatite ; and he says the rock exhibits unmistakeable evidence of extensive alteration. Speaking of its extension eastward, he says “it extends in diminished proportions in a southeasterly direction to the sea at Duporth.”

This mineral occurs in fibrous masses occupying fissures or shrinkage cracks in the serpentine of Duporth, near St. Austell, Cornwall. The thickest veins I have seen are not more than 1 1/2 inches. The fibres are placed transversely across the vein, making generally an angle with the walls of about 70°, so that the crystallization is probably oblique.

The mineral is greenish to brownish gray, has a silky lustre, H about 2 and Sp. Gr. 2.78 : insoluble in HCl, but the iron and magnesia are slowly dissolved out.

Thin fibres are flexible like asbestos. Heated in a matrass the mineral gives off a little water, and becomes lighter coloured ; in forceps thin fibres fuse to a dark glass in the hottest part of the flame. The spectroscope shews the sodium and calcium lines distinctly, but no trace of potassium or lithium.

Mr. Readwin stated that since reading his former paper (Mill Mag., No. 5), he had visited the museums of Copenhagen, Stockholm, Christiania, Kongsberg, and London, in search of corroborative evidences of “Recent Mineral-growth at ordinary temperatures, under ordinary conditions,” and that he was satisfied that mineral “growth under ordinary conditions,” is not of unusual occurrence, .although, hitherto, neither generally observed, nor believed.

Mr. Readwin also stated that several of the specimens referred to in his former paper, some of which he took on his journey, exhibited changes and developments during his absence.

In conclusion, Mr. Readwin submitted an idea of Metal-growth more closely at home.

The Silver-mines, near Kongsberg, in Norway, are said to have been worked unceasingly for the last 200 years or more.

On June 6th, last, I was fortunate in being allowed to visit these celebrated mines, and also the silver-works at Kongsberg. More fortunate still, in obtaining the present beautiful specimens of native silver and Argentitc. They had very recently been taken from the mines, and I was told that my visit was opportune, because such specimens were not of every day occurrence. Be this, as it may, they are of especial interest just now, as illustrating the theory of “metal-growth,” and the more particularly so, as two of them have most obligingly, since they came into my possession, thrown out sundry hints of bodily activity. In fact, have “grown”; and, as they have a direct bearing upon a previous paper, I propose to describe them as well as I can.

When investigating the constitution of antunite or lime-uranite, I secured an extremely fine specimen (of what was then supposed to be this mineral) from the mine at Falkenstein in the Saxon Voigtlaud. I was puzzled by the low percentage of water and other anomalous results which my analysis of the specimen in question gave. While at work upon the solution of this difficulty, Weisbach's note upon uranocircite or baryta-uranite appeared, Winkler's analysis of the Falkenstein mineral clearly proves that we have in it a pure baryta-uranite—a new species of the uran-mica group. I did not, however, abandon my own researches into the nature of this mineral, for the mode in which the water was held by this mineral remained unknown. If isomorphous with autunite, it might be expected to contain 10 aq. in the molecule, 2 of which would probably be retained at 100° C.

It often happens in mineralogical inquiries that from scarcity of materials, lack of time, or other causes, the important datum of specific gravity is not secured. Now, however, that Sonstadt has shown us how to prepare a nearly colourless and perfectly transparent liquid with a density three times as great as that of water, there will generally be no difficulty in ascertaining the specific gravity of very many rare minerals as to which our knowledge remains imperfect. More than this, two minerals if of different specific gravities (provided one is less than S.G. 3.0), may often be easily separated from one another if they are in fragments of appreciable size. Thus, pieces of quartz with a S.G. of 2.655 float, while fragments of beryl (S.G. 2.691) sink in a solution having S.G.=2.67. A very simple apparatus is required for the purpose. (See fig. 7, Plate. VIII.)

Under the name of Romeite a mineral from Borneo has found its way into many collections. There is, however, little in common between the true romeite of St. Marcel in Piedmont and the so-called romeite of Borneo. Both are hyacinth or honey-yellow with darker portions verging towards clove-brown; both contain antimony ; here all resemblance ends. The easy solubility in acids, and the low specific gravity (under 2.0) of the Borneo mineral at once separate it from romeite; it is, in fact, nothing but a variety of allophane containing a little antimonious oxide. The following analysis may not be without interest; the mineral was selected and purified by mechanical means as far as possible, but still contained a small quantity of gangue, chiefly insoluble silica.

One of the blast furnaces at the the Eglinton Iron Works, Kilwinning, having been blown out, it was noticed that on the hearth there was a curious deposit of a bluish black semi-metallic colour, which by its brittleness seemed to be a slag. Mr. Smith having forwarded to my friend Mr. John Young, F.G.S, a sample of the substance, it was submitted to me for analysis and examination. On a cursory examination it was seen that it was not homogeneous but consisted of different substances mixed, the most conspicuous being the bluish substance with a plumbago lustre before referred to. On powdering it the substance was round to contain pieces of metallic iron in irregular masses inbedded in the matrix.

A tendency to assume crystalline forms is a property of all crystalloids ; its full or imperfect development depends on external circumstances.

Crystals produced in laboratories are generally inferior to natural crystals of the same substances, prohaby because of their too rapid formation. The faculty of crystallization is modified by specific dispositions inherent to the substances. There are many distinct types of crystallization, connected with each other by intermediate types, and even with a transition from amorphous to crystallizable substances.

The degree of distinctness of crystals constitutes a fundamental difference. The crystals of many substances are distinct, while below a certain limit of size ; but when this is exceeded, the surfaces are interrupted by re-entering angles, or by irregular drusy cavities or excrescences constantly re-appearing when cleft away, so as, at last, to completely destroy their individuality and regularity.

In 1861, I described (Edinburgh New Philosophical Journal, and American Journal of Science) a borate, found with glauber salt in gypsum at this place, as a new mineral, by the name of Cryptomorphite, on account of its amorphous appearance to the naked eye being resolved into one most distinctly crystalline, under the microscope.

The first criticism on the mineral was made by Prof. Geo. J. Brush (Tenth supplement to Dana's Mineralogy), who said, in effect, that it could not be accepted and named as a new and distinct hydrated borate of soda and lime, because there were already too many names in use for minerals, not clearly made out to be distinct, having more or less closely similar composition.