Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/357

1871. insensible gradations, into the variegated clays and sands of the Wealden. The same is described as taking place in the section (now concealed) of the Ridgway Cutting by the Rev. Osmond Fisher. It is true that Professor Forbes pointed out that, if we compare the shells of the Purbeck with those of the Wealden, we shall find very few species that are common to both. When, however, we consider the fact that the great majority of the Mollusca and Entomostraca which are regarded as typical of the Wealden have been obtained only from the higher portions of the series, the fact will be seen to have but little significance. As is well known, the Purbeck (like the Punfield series) includes certain beds which are of a decidedly marine character and contain shells undistinguishable from common Oolitic species, with an Echinoderm (Hemicidaris purbeckensis, Forbes) which, though not occurring in the marine series of this country, is found in the Upper Oolite of the Jura. Not only is the passage of the Portland into the Purbecks a gradual one in the typical country of these formations, but, as Mr. Godwin-Austen has shown, we find at Swindon Purbeck beds actually alternating with the Portland rocks.

As no break has ever been shown to exist in the succession of Wealden beds in the south of England, we are compelled to conclude that they represent the whole of the vast interval between the Upper Oolite and the Upper Neocomian.

1. "Urgonien" and " Rhodanien" of France, Switzerland, &c.—That the strata known as " Lower Greensand " in England represent the " Aptien " or upper division of the great Neocomian system of Continental authors has long been recognized by geologists. The very careful study of the fossils of the different portions of the Neocomian by Pictet, Renevier, and other palaeontologists has established this correlation in the most satisfactory manner. The beds of white limestone, crowded with shells of the order Rudistes (Chama, Anomia, &c), and with corals, which constitute the " Urgonien " of D'Orbigny, and are so widely distributed in Southern Europe, have everywhere been recognized, both on stratigraphical and palaeontological grounds, as constituting the middle portion of the Neocomian system. In the year 1854 M. Renevier, of Lausanne, who has devoted so large a portion of his studies, with the most valuable results, to the beds of this age, showed that between the " Aptien" and "Urgonien" another series of beds with a distinctive fauna was recognizable, for which he proposed the name of " Rhodanien", To this subformation M. Renevier, after a careful personal examination of the strata and their fossils, assigned the "Perna-beds," "Atherfield clay," and "Crackers " of the Isle of Wight, and the well-known "Couche rouge " of Wassy, in the Department of the

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