In my address at the Annual Meeting of the Mineralogical Society last year, in Plymouth, I gave an account of a new method of studying the optical properties of minerals, and of determining their indices of refraction. I then showed that an approximate knowledge of these characters would, in many cases, enable us to identify minerals with great confidence, and also would sometimes furnish us with valuable hints as to their general chemical composition. My remarks had reference exclusively to detached specimens of more or less considerable thickness, and I scarcely thought that it would ever be possible to apply this method successfully with comparatively high powers in the case of thin sections of rocks. I feared that certain errors, which appeared to be unavoidable, would then become so great in proportion to the difference in the indices of many of the commoner minerals, that it would be impossible to distinguish them with confidence.

Some time since Capt. Marshall Hall called our attention to Arolla, in the Pennine Alps, as a locality interesting to mineralogists. I venture, then, to offer you a brief note on one-of its rocks, collected during a visit in the summer of 1875.

At the head of the Combe d' Arolla rises the grand mass of the Mount Colon, a truncate cone of precipitous rock, dominating the glaciers of Vuibez and Arolla, which, after sweeping down from the watershed of the Pennine chain, unite into one stream at its base. The highest point of its snowy cap is 12,264 feet above the sea. In the Swiss geological map the greater part of the mountain is coloured as gabbro, and the same rock forms a small peak near the head of the Otemma glacier on the S.W., and the S.W. flank of the Dents de Bertol to the N.E.

The chapters of this work purport to be chronicles of the present state of our knowledge of the mineralogy of the northern portion of the kingdom. The geognosy and lithology are treated of to such an extent only as, in the first place, to explain the position of the minerals ; and, in the second, to aid future investigators in their explorations.

In these chapters, in order to do full justice to the work of others, the authority for, and the original discoverer of every locality mentioned, will be quoted, wherever known.

The author, having attempted to visit every known mineral locality to the north of the Forth and Clyde, and having succeeded in this attempt with less than half-a-dozen exceptions, is able in most cases to authenticate, or the opposite, as the case may be, the statements of previous observers.