ALBERT

Beschreibung: ON the advice of Prof. L. W. Collet, of the University of Geneva, M. E. Bovier has, for several years now, been studying the Liassic deposits in what he terms the “second range” of the Jura Mountains, north of Bellegarde in the Department of the Ain, France. He collected a large number of ammonites in the quarry at Prébasson, near Champfromier, where the Upper Sinemurian is well developed, especially deposits in the oxynotum zone. The stratigraphical details and full lists of fossils have now been published by M. Bovier in a paper entitled “Les Ammonites du Simémurien supérieur de Champfromier (Jura français)”. The object of the present note is merely to direct attention to some facts of importance for the correlation of Liassic deposits, facts that confirm the suspected unsoundness of polyhemeral Liassic chronology. The whole problem of correlation over wider areas is being discussed more fully in connection with higher beds in the Jurassic. M. Bovier compared his ammonites with the Liassic material in the Geological Department of the British Museum, collected in the corresponding Upper Sinemurian beds at Charmouth. For it was Dr. Lang's collection, in the first instance, being the only material available that had been carefully collected, on which the original subzones (within the obtusum zone) were based, as well as the later subdivisions of the oxynotum and raricostatum zones. I ought to make it clear in this connection that the term zone is here used in its original meaning. Thus d' Orbigny in 1850 already spoke of his Kimmeridgian stage as the “zone of Ammonites lallieri, of Ostrea deltoidea and O. virgula,” while Oppel, six years later, wrote of “zones” or “horizons” as being distin uished by the constant and exclusive occurrence of certain species. Therefore the zone is defined by the range or vertical distribution of species (or better an assemblage of organic remains), irrespective of stratal or lithological incidentals. Sub-zones consequently are smaller divisions of geological time, characterized by the local restriction of certain index species. The value of these local subzones for purposes of an ideal time-scale (Oppel's “ideales Profil”), in my opinion, is nil. If the Champfromier sequence had been worked out before that at Charmouth, the order of Upper Sinemurian “hemerae” (rashly adopted in some textbooks) would have been very different. It is clear that the restricted occurrence of any particular ammonite at a given locality is something entirely different from the actual date of existence (range or hemera) of this species.