Of all the Gault fossils, the Ammonites take precedence both for beauty and number; and, holding as the Cephalopods do the highest rank of molluscous animals, we cannot but view their extraordinary variety in this deposit with singular interest. If, therefore, we dwell here somewhat at length on their general classification, we could scarcely choose a more fitting occasion.

As ordinarily seen in collections, the Gault Ammonites appear to be all more or less of diminutive size. Few exceed two inches in diameter; many are little more than an inch; while one of three inches would be looked upon as a fine individual.

These specimens are, however, for the most part only the central whirls solidified or hardened by interior walls, or in-fillings of iron-pyrites. In the Gault itself Ammonite-shells of far larger size—commonly of six or seven inches across, sometimes more than a foot—are not merely frequent, but extremely abundant, although, from the fragile state of the main mass, they are commonly cleft to pieces in breaking out the central pyritous cores without attracting observation.

The inferior division of the Carboniferous series in Derbyshire and North Staffordshire, composed of calcareous rocks and shales, and forming the Mountain Limestone group of those counties, presents, particularly about the neighbourhood of Alstonfield, some very interesting and remarkable associations of metallic minerals with certain mechanical disturbances of the strata; moreover, these associations are not to be indiscriminately classed with the general phenomena of the mineral veins of the districts in question, but must be considered as special facts requiring a separate consideration and explanation. Their existence is no new discovery, since they have been recognized from the earliest times that mines have been worked in the places where they occur, and where also they are at most times regarded as valuable features, inasmuch as the richest deposits of ore have been found in connection with them; indeed, so decidedly has this been the case, that, in working the mines, much of the future success has been calculated by the amount of probability of any particular vein intersecting the disturbed beds; such unions having, in nearly all cases, been attended with most important results, both as respects the quantity, as well as the kind of ore met with. Nevertheless, I am not aware of their having ever received any particular attention from geologists, nor of their having been anywhere described, circumstances which may probably arise from the fact of these beds only being visible below the surface, and usually in deep mines; or they may have remained unnoticed, from the really small amount of scientific observation which in this country has been brought to bear on the facts connected with metalliferous deposits, compared with what has been done, and is still doing, in other branches of physical geology. The present article is drawn from memoranda made during several careful surveys of mines which are notable, in North Staffordshire, for exhibiting the phenomena of the saddles in great force.

The first person to verify the geological age of the flint-implement strata of the valley of the Somme was Dr. Rigollet, who in 1854 published his “Mémoire sur des Instruments en silex trouvés à Saint Acheul près Amiens, et considérés sous les rapports Géologique et Archéologique,” illustrated by five sections of the beds oy M. Dutilleux, and with drawings of the worked flints. After stating that on many occasions the bones and teeth of fossil elephants had been met with in the same beds, he adds—“My curiosity was strongly excited in the month of August last (1854), when M. Dutilleux, Member of the Society of Antiquaries of Picardy, informed me that he had found there also axes or instruments of flint evidently worked by the hand of man. This fact, however astonishing, was the less so as M. Boucher de Perthes had announced the like discoveries at Menchecourt and at the mill at Quignon, near the gates of Abbeville.

Feeling that these discoveries supported the statements of M. de Perthes, Dr. Rigollet thought the geological question the most important, and the first to be investigated.

In concluding this paper, allow me to remark that if the deductions to which I have arrived be correct, some data will have been afforded for explaining, through the agency of analogous chemical changes and their resulting products, the cause of some, at least, of the various distinctive characters presented by those rocks which constitute that portion of the earth's surface formed from the decomposition of previously existing rocky masses. I will not, however, take up your time with any lengthy arguments to strengthen my position in such a manner, but will merely attempt to give a very brief description of what I conceive we may not unreasonably infer has taken place during past ages of the world's history, remarking on the various geological formations in the general order of their occurrence; and, firstly, I would direct attention to the important bearing the chemical changes described may be presumed to have on the solution of the question as to the geological equivalence of the Old Red Sandstone rocks of Scotland to those of our Devonian era. I have before adverted to the occurrence of red sand in a decomposed slaty seam of the Plymouth limestone, and would add that such red sand is a frequent result of the decomposition of its dolomite, and that sandy beds of a similar kind are also not unfrequent in the limestone itself. If we call to mind the fragmentary condition of the fossils of the Old Red Sandstone strata, it may not be considered unreasonable to suppose that they have been formed from the decomposition of rocks similar to those of our Devonian limestones, in which iron pyrites was much more abundantly distributed.