1923 CHRONOLOGY 31 



morphic equivalents : there should be a general parallelism of species 

 with only an occasional foreign element. Just the reverse of this, 

 however, is the case. The differences between the species are greater 

 than should have been developed in isochronous strata of two provinces 

 only lately separated. Therefore the conclusion is reached that homo- 

 taxial strata of the two areas are not isochronous — the difference of 

 species is chronological, not geographical. This is the idea of the second 

 theory' — that the Ammonite-faunal differences between England and 

 Wurtemberg during the Kimmeridgian are due to alternating pene- 

 contemporaneous erosions in the two areas, the preservation in Wurtem- 

 berg of strata which England has lost, and the preservation in England 

 of strata which Wurtemberg has lost. 



It is in favour of this theory that it accounts for such phenomena 

 of faunal dissimilarities in contiguous parishes where there can be no 

 reason for supposing difference of province ; for quite small areas would 

 be a mass of little provinces if barriers were erected for all faunal 

 dissimilarities. The case of the Kimmeridgian strata of England and 

 Wurtemberg differs only from that of the local Portlandian strata, 

 to which the theory of denudation is especially applicable, by the 

 greater distance involved ; yet prior to Cardioceratan such distance was 

 not too great for faunal similarity. 



The hemeral sequence given in Table III, p. 33, is based, therefore, 

 on the theory that great faunal dissimilarities are more likely to indicate 

 difference of date than difference of province. Hence, homotaxial, but 

 strongly anisidophorous strata are considered not to be synchronous. 

 But to find the correct sequence is a difficulty. The more anisidophorous 

 the strata, the greater the argument for their anisochronism, but the 

 greater the difficulty of a true sequence. Therefore Table III must be 

 regarded as an approximation — an outline scheme of dating to be utilized 

 and amended : some of the evidence on which it is based will be given. 



The advance in the number of divisions in the Kimmeridge Beds 

 within a few years is noteworth}-. In 1895 H. B. \\'oodward said : 

 " There is no need in this country [England] to divide the Kimeridge 

 Clay into more than two zones for general stratigraphical purposes, 

 and these are intimately blended " (Jur. Rocks, \, 152 ; Mem. Geol. 

 Surv.) ; but he parted these two zones into three sub-zones. In 1913 

 Dr. Salfeld (Tab. i, pp. 128-130) made ten zones for approximately 

 the same strata, giving only one doubtful gap in the British sequence. 

 Now, in 1923, a chronological sequence of over forty hemerje is proposed, 

 \vith the suggestion that only a httle more than half of the deposits 

 made during these hemerae have been preserved in Britain. The rest 

 are supposed to have been more or less completely removed ; but exposure- 

 failure, collection-failure and (faunal) preservation-failure may have 

 exaggerated this supposed loss of deposits. 



When the possible thickness of Kimmeridgian deposits is taken into 

 consideration, the demand for forty or more hemerse as the length of 

 time in which they were laid down does hot seem so excessive. For 

 instance, Choffat (Amm. Lusit. ; Mem. Trav. Geol. Port. 1893) gives 

 for his Lusitanian (about Aulacosphinctean to Cardioceratan inclusive), 

 an approximate thickness of 5,500 feet. He divides as follows : — 

 3. Assise d' Abadia . . . . about 2,400 feet 

 2. Calcaires de Montejunto ,, 1,500 ,, 



I. Calcaires de Cabago . . ,, 1,500 ,, 

 The beds of Abadia and Montejunto are approximately equivalent to 



