The two Memoirs by Dr. Reynès, on the Cretaceous formation, which were noticed in the ninth number of the Geological Magazine, suggest two questions: how far 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 l'Étage,’ involves considerations which might be well extended 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.