For many years English palæontologists were content with the limited number of Portlandian Gasteropoda described by Sowerby—principally from internal casts—in Fitton's celebrated paper on the “Strata below the Chalk.” Other authors have described a stray species here and there: still the list was scanty, and not only scanty but indefinite; and many ingenious guesses have been hazarded as to the true affinities of such species as Buccinurn angulatum, and Buccinum naticoides.

While I was engaged in surveying the Drift-deposits of the neighbourhood of Warrington, Runcorn, and the Peckforton Hills near Chester, my attention was called to what appeared to be a good boundary-line in the middle of the Lower Keuper Sandstone, separating the Waterstones from the Conglomerates underlying them. The sections at Runcorn and Frodsham, where I first saw this line exposed, have been already described with many others in the Survey Memoirs on the Neighbourhood of Prescot (3rd edition) and on the Neighbourhood of Chester. They showed that there was no passage from the Waterstones down into the Conglomerates, but that there was on the contrary a sharp division between them; and as on following this line further south, I found that it was persistent, and everywhere separated beds of a very different nature, it was decided that it should be engraved on the Map, the new classification of the Keuper of this neighbourhood, as contrasted with the old, being shown in the accompanying figure.

In a previous paper we showed that the extinction of the Mammoth was sudden, and was accompanied by a sudden change of climate throughout Northern Siberia, which enabled its soft parts to be preserved intact. We did not mean that the change of climate was the cause of the Mammoth's extinction. This it clearly was not, for this change on a large and marked scale was apparently confined to Siberia, while the Mammoth disappeared from a much wider area, where this climatic revolution could not have been so fatal. A sudden change of climate could not account for a catena of Mammoth carcases found buried several feet underground from the Obi to Behrings Straits. If they had been killed by the cold merely, their bodies would have lain on the surface where they fell and become long ago the prey of the predatory animals. Nor again can we, with such a postulate merely, account for the remains occurring in many cases in hecatombs, and in many cases also with a mixture of various species of animals. The cold would hardly have destroyed the bears and hyænas which are found linked in a common fate with the Mammoths and Rhinoceroses. Nor, as we shall see, would a mere change of climate, however severe, account for other factors in the problem. The change of climate accounts perfectly for the preservation of the carcases, which preservation again necessitates the conclusion that that change was a sudden or very rapid one; but we have clearly still to seek the real cause of the disappearance of the animals of which the change of climate was a concurrent effect.

It has been for some time a matter of surprise to me that Dr. Holl, in his exhaustive memoir “On the Rocks of S. Devon and East Cornwall,”—after once raising the question “whether the limestones” (those of our present subject) “might or might not be the same as those of Ogwell, Ipplepen, and Dartington thrown over a broad anticlinal axis of the lower slates to the North-West,”1— should have ceased following out that line of thought to what appears to me the only possible issue, since he thoroughly recognized the existence of uniclinal structures in parts of the district.

“It is often said by vein-miners who have worked at great depths, that the jointed structure of rocks fades away and disappears in the deeper parts of the mines. This, however, is probably a case of the rule ‘de non apparentibus et non existentibus eadem est ratio.’ All the joints near the surface are more or less acted on by the weather. The deeper-seated rocks may be just as much traversed by joints, but they are merely mathematical planes of division, the faces of the blocks adhering as closely as if they did not exist till the weather makes them apparent, or some force tears the blocks asunder.”

The mineral composition of rocks, often an important test in comparatively unfossiliferous formations, such as the Trias, becomes of supreme value amongst the Archæan groups. It is very frequently the only kind of evidence available, and, used with due caution, may lead to decisive results. In Anglesey, as illustrated in Fig. 1, and already described, it is nearly as conclusive as the testimony of organic remains; for the great similarity of the dark schist in two areas so near together renders their correlation in the very highest degree probable. The gneissic rocks of the Malvern Hills have been classed as Laurentian on the ground of the lithological resemblance between the two groups, though 3000 miles apart; and the determination has been generally accepted. But in such a case as this, high probability is all we can expect. Other gneissic and schistose systems have been described in America, though none of them so closely resemble the Malvern series as the Laurentian.

As the Archæn geologists have turned their attention to the Old Irish rocks, perhaps it may be allowable to give a short description of them.