In the definition which I have given, a mineral is described as a body possessed of a definite chemical composition and a regular physical form,—meaning by the latter particularly crystalline structure. But, while the crystalline relations of each species only vary within the narrowest limits, the chemical composition has a much wider latitude. Many individuals of the same mineral species are found, by analysis, to differ most widely in their chemical components, while the crystalline form is quite unaltered. Now, although we are as yet unable to trace the relation between the form of a mineral and its composition, there can still be little doubt that some such relation does exist, regulated by laws yet to be discovered; consequently, when we find the same mineral species differing widely in chemical proportions, we recognise a departùre from regular although unknown laws, and seek for a cause. This we find in the doctrine of Isomorphism. This, simply stated, in the capability of two or more substances, of analogous chemical constitution, to crystallize in similar forms.

During the last few years, the native iron-making resources of South Wales have received considerable assistance from the re-discovery and working of some very remarkable deposits of hæmatite in the county of Glamorganshire. I propose in the present article to describe two of the most important, perhaps, of those veins of ore, which are found in the districts of Llantrissant and Llanharry, near Cowbridge, and at Newton Nottage, near Bridgend;—the iron-ore in the latter locality being associated with a very large and curious deposit of manganese, chiefly psilomelane. Prefatory to this description, it may not be out of place to give a few remarks on the physical geology of the surrounding country, inasmuch as this district possesses ample materials to engage the attentive and careful examination of the geologist, particularly in relation to the origin of the mineral deposits in question. The exploitation of these hæmatite-mines will have the effect, commercially, of giving a great development and increased prosperity to the iron-manufacture in the district south of Cardiff; while, as a social result, their being worked will probably bring back to a population, at present agricultural, the mining and metallurgical occupations of the ancient fore-dwellers on the soil, and, what is most desirable, give constant occupation to a very large number of the working classes.

Much attention has been excited in France for some few months past by the fall of a large aërolite, which took place in the canton of Montrejeau on the 9th of December last. We have now all the details that we are ever likely to have concerning this remarkable meteor. It fell about seven o'clock in the morning, appearing first in the north-east like a large red-hot bomb, which passed rapidly to the south-west, where it remained stationary for an instant. It then emitted a considerable column of smoke and flame; three seconds after which a loud detonation was heard, followed by a rumbling noise. Although in broad daylight, the little town of Aurignac was completely illuminated by the passage of this aërolite. After the explosion nothing was observed in the sky but a streak of vapour and a small cloud which marked out the direction followed by the meteoric stone, and the place where it exploded. Shortly after this phenomenon two large fragments of the aërolite were picked up in the parishes of Aussan and Clarac; one of these weighed about 90 lbs., and had sunk into the ground for nearly two yards; the other, that fell at Clarac, broke through the roof of a cottage; it weighed from 16 to 20 lbs., and was so hot when first seen that it could not be touched for some time. These blocks present rounded forms, their surface is black and smooth, the interior is formed of a sort of grey substance, not unlike certain volcanic products in structure.