This unusual substitution appears to have taken place in a rock found by Mr. R. N. Worth, F.G.S., on Trowlesworthy Tor. The specimen was a loose block, and the rock to which Mr. Worth has given the name of Trowlesworthite has not yet been discovered in situ. The rock consists of red felspar, fluor-spar, and tourmaline, associated with quartz, and its general character and microscopic structure are described in papers published in the Transactions of the Royal Geological Society of Cornwall, Vol. X. p. 177.

The Harz Mountains rise from the great plain of secondary and tertiary formations in the western part of Prussia, about 130 miles inland from the seaboard at the mouth of the Elbe. Like the Thüringer Wald and other adjoining ranges, the Harz trends in an approximately east-south-east and west-north-west direction, and resembles on the map an elongated ellipse with a northern flattened side some 50 miles in length. It has been compared to a single mountain with large shoulders, culminating in the celebrated Brocken, 8,746 ft. in height. A line drawn through the Brocken from north to south roughly divides the region into two parts.

When I submitted my Paper on Enargite (Min. Mag. Vol. vi. p. 49) to this Society, I was unable to give the exact locality of the mineral. As the territory of Montana embraces a considerable extent of land, I was therefore desirous of ascertaining the position of the mines yielding the ore.

Having during the past summer had the opportunity of visiting the locality, I am now able to give some further particulars.

My attention was first directed to this rock by Mr. Cruickshank, clerk of works at Beaufort Castle, in July of last year, when, from the description he gave of it, I ventured to pronounce it barytes. Specimens were soon after this put into my hands by Mr. Cruickshank, when I found that my conjecture was a correct one. There was another mineral associated with the barytes, of which we will speak afterwards. The specimens received were so very fine, that I resolved to visit the locality to be able to describe its surroundings and procure specimens for myself. This I did in May last in company with two of my colleagues and Mr. John Ross, one of Lord Lovat's gamekeepers, and who I believe was the first to call attention to this new locality.

The deleterious effects of small quantities of Arsenic when present in Copper are weI! known to all who are concerned with the working of thin latter metal, and the elimination thereof is consequently an object to which the copper smelter pays special care. Though a large number of minerals, some of which are of exceeding beauty, are found composed of Copper combined with Arsenic acid, the direct combinations, as Araenide and Sulph-Arsenide, are comparatively rare.

The crystals are of a similar length, though often considerably thicker than those described by Prof. Maskelyne (Phil. Mag. Jan. 1868), and his general description applies to those under consideration.

They are, however, ill-adapted for measurement, as single perfect crystals are rare. As a rule several crystals are longitudinally attached to one another, with slight radial divergence, in such a manner that it becomes difficult or impossible to discriminate between the reflections of their minute faces.

In this Paper the author directs attention to some localities in Scotland where the aetinolite forms of hornblende are to be found.

In Aberdeenshire the actinolite occurs as sheaf-like groups, irregularly distributed through a felspathic base ; also as slender crystals, extending sometimes to the length of eighteen inches, in a dioritic rock. On the hill of Heshgarnich, in Perthshire, this variety of hornblende occurs in nests, also in fan- or sheaf-like bundles of a black colour ; and on the north side of the hill it is found in blade-like forms distributed through a glistening micaceous schist.

Perhaps the most difficult question in the science of Geology at present, is the explanation of the origin of the metamorphic crystalline rocks which occupy large areas on the surface of the globe. These rocks have been generally regarded as having undergone a vast amount of change in lithological character, to which the term metamorphism has been applied. By local or contact metamorphism is meant the transformation which has been effected in ordinary sedimentary strata by the intrusion of igneous rocks, the extent of which depends on the relative size of the intrusive mass.

The proportion of actual carbon varies much in different samples. Some portions are stated to have analysed as high as 75 per cent., but, as will be noticed from the figures given below, the carbon may be much lower.

During the past few years the study of the minute structure of minerals and rocks has received the attention of so many able geologists, that microscopical mineralogy and petrography may now be regarded as a fairly established means for the identification and diagnosis of inorganic species. As a special instance, I may point to Heddle's researches on Amazonstone, where the discrepancy of its chemical constitution from normal orthoclase, and its aberrant crystalline form, are accounted for under the microscope, with polarised light, by minute structural peculiarities which support the view that it is composed, not of orthoclase and albite, as has been supposed, but of orthoclase and a paragenetic constituent, allied to, if not identical with, oligoclase.

This body is a hydrocarbon which has been classed under the Asphalt group ; but, unlike Asphalt, it does not melt at 100°C., nor is it at all soluble to the extent Asphalt is in such solvents as ether, oil of turpentine, benzol or carbon disulphide. When heated in a closed tube it shows incipient melting after intumescing, but, at the same time, it decomposes into its constituent gases. This chemical change takes place at 300°C. In the above solvents about 5 per cent. of bitumen is obtained. Carbon disulphide softens the mass throughout to a pasty consistence.

It appeared of interest to determine the nature of the alteration which has changed the light greenish or brownish yellow siderite to the dark, almost perfectly black, glossy crystals so often seen on specimens from this neighbourhood.

The material, gathered from the heap of a mine, consisted of massive siderite, crystallised on its natural surfaces in the form of small curved primary rhombohedrons, and associated with quartz, pyrites, blende, and other minerals.

With little else than the trouble of surveying one of the finest of Edinburgh landscapes, the tourist beginning work amongst the débris just to the south of where Samson's ribs overhang the road, and continuing on the road along Salisbury Crags to Holyrood Palace, will meet with a continuous display of the relations specially of zeolitic minerals, and their containing rocks. If he care to pursue the literature of the subject, Daubrée's investigations on the genesis of zeolites, as well as Allport's, Bonney's, and Geikie's petrological researches into the microscopy of the rocks in question, will suggest oven more lengthened studies.

The samples to which this note refers were freshly cut from the quarries at Strontian. The mass differs much in quality, but is largely made up of the usual acicular groups. The colour of the mass as received varied, according to the composition, from a slight green shade where Strontianite predominated, to the more pure white of barium and calcium compounds.

The analyses were made from various portions, No. 1 being the selected Strontianite, No. 2 the more white portion of the mass.

The rocks whose microscopic structure it described in this paper were collected by my friend, M. G. Stuart, Esq., during a tour in Australia, under the kind auspices of A. W. Howitt, Esq., F.G.S.,the well-known geologist, who has so largely added to our knowledge of one of the most interesting districts of that continent. Some of them have been already noticed by that gentleman, but as the volumes in which his descriptions are published are not very accessible to English readers, and as two of them are rocks to which I have devoted special attention, I have ventured to bring them to the notice of our Society.