It is an old remark, that much of the prosperity attending the manufacture of iron in Great Britain has resulted from the circumstance that the ore, the fuel, and the flux are mostly associated minerals in the same locality; and, although this is typically true as regards the mineral basin which forms the subject of the present article, it is some-what curious that, in all probability, the discovery of the fuel in this instance dates much posteriorly to the employment of the ore, so that iron-making, had become an active branch of industry in this district long prior to any of the numerous seams of coal being worked; and, what is equally likely, before even their existence was known. If we look into any of the early histories of Gloucestershire, we find that, while abundant mention is made of the iron-mines and the smelting furnaces, no notice whatever is afforded of the coal, fully warranting the supposition that the discovery of the deposits had not then been made. I t will, perhaps, form an interesting preface to this paper, if, in connection with this latter circumstance, before entering upon the geological and lithological features of the ironstone formations, which are special and of the greatest interest, I give a sketch of the commercial history of the locality, past and present.

In tracing the progress made in the metallurgy of iron in this country, the Forest of Dean claims to have been among the first localities where the manufacture of that metal was established.

It is one of the many striking facts of geology that, as we pass from one great group of rocks to another, we find a considerable change both in lithologioal and zoological characters. Sometimes the transition is abrupt and sudden, at others, slow and gradual. In the former case, the marine fauna of the older deposit had perished, and a new race sprung up and succeeded to its place; while, in the latter, there was only a gradual dying out of some species unable to exist under the changed oceanic conditions, others more hardy still continuing to flourish in the later formation. But this change of genera and species is not confined necessarily to the breaks which seem to have taken place between one great rock-system and another; it is often to be noted in a greater or less degree, as we have seen in our review of the Oolites in this county, in the minor divisions and sub-divisions of each formation. The student must also bear in mind that the strata which form the crust of the globe were, for the most part, deposited by the sea; a very small proportion only being the products of fresh water, and still fewer being purely terrestrial. Therefore, in our researches into the history of the past, we have to deal more usually with the varied inhabitants of the ancient seas, and to mark the changes which took place from time to time as we pass through either entire periods, or the parts and details into which they may be divided.

The Llandovery rocks were so named by Sir R. Murchison from the locality in South Wales where they are most fully developed, and where their physical relations to the formations above and below them are clearly exposed. Those rocks constitute an intermediate place in the Silurian table, connecting, by some of their contained organic remains, the fauna of the upper and lower Silurian groups. The upper Llandovery rock contains many shells common to the Wenlock beds, and is considered to form the natural base of those deposits; on the other hand, the lower Llandovery is related by its fossils to the Caradoc beds below, and both members of this formation are united by having certain fossils in common and peculiar to them, but are chiefly characterized by the abundance of some species of Petraia, Atrypa, and especially Pentamerus. This group consists of soft, argillaceous sandstones and building stones (Goleugoed), shales, some calcareous bands, hard, coarse grits, and quartzose, pebbly conglomerates. Thickness 2,000 to 3,000 feet.

Localities.—Both members of this group, and their relations to the overlying and subjacent rocks, are well exhibited at Noeth Grüg, near Llandovery, South Wales. In Radnorshire, Shropshire, and Hereford, the upper member, or Pentamerus oblongus zone, is alone present, and is seen in some places to be unconformable to the Longmynd and Caradoc rocks. The conglomerates of the west flank of the Malverns, and the sandstones of May Hill and Huntly, Gloucestershire, are of this age.

A Man would see but little of the reality of the world if he shut himself up in his house, and only gazed out from the same window; he would learn little more if he contented himself with alternately gazing upon the scenes passing around him, from the windows of every storey. So a geologist, in limiting himself to the study of the rock-masses of a circumscribed area, would never, by the utmost perseverance, in going continually over the same ground, attain to a perfect understanding of the subject of his study. He must go abroad, either in his own person or equivalently, by making himself acquainted with the travels and labours of others. Our knowledge of the ancient conditions and relations of the oldest rock-masses would not be complete if we limited our investigations to those isolated patches in our own country, which, however important, are still only a part of that great whole, more important traces of which are to be met in regions far away. Thus those very old—indeed, primitive sedimentary rocks, represented in the British Isles in a fragmentary manner, as by the younger or bedded gneiss of the Scottish Highlands, assume in Canada and the Arctic regions proportions of great extent, and consequently, of far greater value. Far back in the obscurity of the past, as must be placed the birth-time of these primitive land-masses, we seem, in our first investigations, plunged in interminable ignorance, like the explorers of some vast subterranean cave in impenetrable darkness.

The Boulder Clay is so seldom exposed in the neighbourhood of London, that I think it may be desirable to point out a new locality where I have met with it and where it may yet be seen, especially as I have just found some points, connected with its position at this place, which are of rare occurrence. The new line of railway from Watford to St. Albans passes chiefly through gravel and chalk. At one place, however, called Bricket Wood, about midway between those towns, there is a cutting of some length, and twenty to thirty feet deep, entirely through the Boulder Clay. The superposition of this clay, with regard to the gravels of the London district, is seldom to be determined; the cutting itself throws no light on this subject, or, at least, nothing very definite. But a ballast pit has recently been opened at the Watford end of the Bricket Wood cutting, and immediately south of the line, which exposes a section of much interest. The Boulder Clay has there almost thinned out, leaving but a seam one to two feet thick, whilst both above and below it is a thick bed of gravel. The lower sandy gravel, which consists chiefly of sub-angular flints and flint pebbles, with some quartz, sandstone, and old-rock pebbles, with subordinate seams of whitish sand, has a clean, washed appearance.