II. On the Classification of the Cretaceous Beds
T HE two Memoirs by Dr. Reynes, on the Cretaceous formation, which were noticed in the ninth number of the GEOLOGICAL MAGAZINE, suggest two questions : how far the subdivisions there proposed are applicable to the Cretaceous series of this country; and, next, as to the sufficiency of the ground on which the synchronism of the subdivisions of geological formations has been based. The subject of the second Memoir, ' De FEtage,' involves considerations which might be well extended to the whole
... ed to the whole range of the geological series : indeed, it will hardly be profitable to make much progress with the past physical history of the earth until rules for determining geological horizons shall be established. The first of these questions has probably been answered much in the same manner by many English Geologists. Things do not admit of subdivision and the comparison of relative parts, unless they have something in common. Dr. Reynes proposes ten subdivisions for the Cretaceous series, each characterized by from three to four forms of Shells or Urchins. Out of thirty-eight species which are cited, as many as eighteen have not hitherto been recognized within the British area; so that twenty species alone serve for comparison, or an average of two for each ' stage.' This is a small amount of evidence. In that school of Palaeontology, however, in which it is a fundamental doctrine that a species has a definite duration in time, and that identical forms, wherever met with, are proofs of synchronism, a single species would have done as well. The twenty species which have to be removed from Dr. Reynes' list belong to the Mediterranean province of the Cretaceous Period, •with which he is best acquainted. Dr. Reynes' highest horizon * is that with Belemnitella mucronala, B. quadrata, Micraster cor-anguinum, and Hemipneustes radiatus : this last, a well-known form from the Maestricht and Gulpen beds, has not been found in England. The three other species may be placed thus :-Belem. mucronata, Micraster cor-anguinum, and Belem. quadrala. These forms, which are abundant in the Chalk of the North of France, of Belgium, and of England, do not occur in the Cretaceous beds of the Mediterranean area; and on such grounds the uppermost division of the series might be supposed to be wanting there. Nor, if recourse be had to considerations which will be noticed in the sequel, can there be any comparison established between the Upper Chalk of England and that of Maestricht. In the horizon next below the second, the forms cited are Badiolites, Spharulites, and Hippurites, with Micraster brevis and Ostrea auricularis. M. brevis may be only a Mediterranean-area variety of M. cor-anguinum ; but the form does not occur with us, nor in the * Upper Chalk-Chalk of Maestricht and Meudon.