Ammonite Beds Research Articles

The Gastropoda and Lamellibranchia of the Green Ammonite Beds of Dorset

I. Introduction. Most of the mollusca here described have been collected during the past 30 years by Dr. W. D. Lang. Older material in the British Museum collection shows that the Green Ammonite Beds had also occupied the attention of such collectors as the Rev. P. B. Brodie, R. F. Tomes, and (especially) T. J. Slatter, but the exact horizons of their specimens are unrecorded. Of previous published records of non-cephalopod mollusca from these beds, reference need only be made to R. Tate's description (1870 a , p. 404) of four small gastropod species, Straparolus wrightianus, S. bellulus, S. aratus , and Littorina biornata , obtained by H. B. Brady in washing material collected by T. Wright for foramintfera, and to a short list published by E. C. H. Day (1863, p. 291), which includes the two forms Chemnitzia carrucensis [ sic ] d'Orbigny and Pterocera liasina d'Orbigny. The first of Day's records ( Chemnitzia carusensis d'Orbigny) may or may not be correct; the species is not in the material examined by me. His record of P. liasina is meaningless, as this form has been ignored by French authors since its original, and totally inadequate, description in d'Orbigny's Prodrome , and the type is no longer extant. 1 The most notable feature of the non-cephalopod molluscan fauna of these beds is the abundance of small gastropods, particularly a few feet above the Belemnite Stone, and of Taxodont lamellibranchs belonging to the genera Nucula, Nuculana , and Parallelodon . Most of the lamellibranchs and some of the gastropods are long-ranging forms. A

The Green Ammonite Beds of the Dorset Lias

I. Introduction. The Green Ammonite Beds comprise those horizons of the Dorsetshire Lias which lie above the Belemnite Stone—horizon of Beaniceras —and below the Three Tiers—nearly the lowest limit of Amaltheus . They bear this connotation in the memoirs of the Geological Survey, and include a greater thickness than was intended by Day (1863, p. 291), who first coined the term for 18 feet of clays lying in the lower part of the series as here defined, and yielding nodules in which the typical green ammonites are to be found. The chambers of these ammonites are often filled with greenish calcite—hence the vernacular name. The Green Ammonite Beds form part of the Upper Lias Marls of De la Beche's earlier account (1826), and the middle part of the “marls and slaty marls with several beds of indurated marl and earthy limestone in the lower part, micaceous in the higher—350 ft.” of his later account (1839, p. 223). They are the equivalent of Oppel's “Davöi-bett”, or “zone des Amm. Davöi ” (Oppel, 1856–8, pp. 117, 126–9, 148, 151) and he records their occurrence on the Dorset coast (p. 129) as follows: “An der Küste von Charmouth bei Lyme Regis (Dorsetshire) fand ich dagegen nicht allein ein grosses und deutliches Exemplar von Amm. Davöi [also recorded on p. 161], sondern in seiner Begleitung auch die in Schwaben mit ihm zusammenliegenden characteristischen Arten: Amm. capricornus, Henleyi [also recorded on p. 164], Belemnites umbilicatus [also recorded on p. 154], elongatus [also recorded on p. 152], clavatus [also

On the Contemporaneity of Certain Ammonite Beds in England and France

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.

On the Liassic Succession of Pabay, Inner Hebrides

It is intended to give in the present paper the results of the writer's collecting in the Lower Liassic (Carixian = Lower Pliensbachian) Ammonite Beds of the Island of Pabay (Pabba), known to fame since MacCulloch and Hugh Miller drew attention to the abundance of its fossils and Murchison and A. Geikie collected its ammonites. Some of these were many years ago described by Sowerby and Wright, and in the more recent Survey Memoir1 a list of fourteen ammonite species was given, a number trebled in the present account. Unfortunately, the hundred feet or so of micaceous, metamorphic shale at Pabay is cut up by “dolerite and basalt dykes and sills” to such an extent that the tracing of the succession is not an easy matter.

IV.—The Lower Pliensbachian— ‘Carixian’—of Charmouth

The Green Ammonite Beds of the Lias (called also the Wear Cliff Beds in the latest Survey publication on Lyme Regis) include the clays with occasional limestones that lie between the Belemnite Stone below and the lowest of the Three Tiers above. The lowest Tier contains ammonites of the margaritatus group, and the Belemnite Stone caps the top beds of the Belemnite Marls that may be placed in the ibex-valdani zone. So the Green Ammonite Beds may be said to constitute the upper part of such of the Pliensbachian or Charmouthian as lies below the Domerian. Buekman has applied the term ‘Charmouthian’ to this lower portion, thus restricting the term to a part only of what it originally included. I have already advocated the propriety of applying the term Charmouthian strictly with its original connotation, and would call those zones of it that lie below the Domerian,—Bonarelli's ‘Charmoutiano inferiore’—which certainly need an inclusive name, Carixian. The fullest published accounts of the Green Ammonite Beds are those in the Survey Memoirs, and may be summarized as follows: They extend from Black Ven on the west to Seatown on the east; traces only appear on Black Ven; the complete series occurs on Stonebarrow, though there “only the lower portion is well-exposed”; their thickness is about 100 feet, though variable, and as much as 125 feet to the east of Golden Cap; they consist of marly clays with ‘ferruginous bands’ and nodular limestones.