24 TYPE AMMONITES— IV Dec. 



areas within the Hmits of erosion, whereby penecontemporaneous denuda- 

 tion produced, in any province, faunal dissimilarities, marked by local 

 absences of faunas of certain hemerae. 



It may be urged that the evidence for such crustal movements 

 would be found only in strata laid down in shallow seas. If that be so, 

 a deep-sea deposit, like the Tithonian, should have no faunal failure ; 

 but, if it contain a complete faunal sequence, there should be in its fauna 

 something analogous to the faunas of the Behemothan-Gigantitan Ages 

 of England. These faunas are very local in Northern Europe, and the 

 phenomenon of their absence is explained by the hypothesis of pene- 

 contemporaneous erosion. Is it necessary to have another hypothesis 

 to explain their absence from the Tithonian ? 



It does not seem reasonable to suggest that crustal movements 

 were local, and happened only where seas were shallow. It seems more 

 justifiable to suppose that crustal movements were like the waves of 

 the sea, continuous, widespread and of variable magnitude, able in time 

 to raise even deep-sea formations to within reach of denuding agencies. 

 Time is the factor for which insufficient allowance is made. A hemera, 

 though taken as the chronological unit, must be regarded as a very 

 lengthy stretch of time. Migration of Ammonites would be a slow 

 process ; but, in comparison with net accumulation of deposition of 

 strata, it would be so rapid, or the latter was so much slower, that the 

 accumulation of deposit was insufficient to mark the point of faunal 

 departure from the point of arrival. The rate of Ammonite migration 

 to that of deposition was like the flight of an aeroplane to the progress 

 of brick-laying. 



Present-day phenomena of deposition or of faunal dispersal are 

 very unsafe guides. Geological strata are made by the net result of a 

 constant battle of addition versus subtraction, in which are seen, locally, 

 the small, slow victories of addition, after many vicissitudes. 



The same arguments apply to modern faunal irregularities — they 

 cannot be true criteria of what the ultimate geological record in the 

 rocks will be : they are only records of temporary local phenomena, 

 observed during a length of time quite negligible in comparison with 

 the length of a hemera. 



Detailed hemeral sequences will illustrate the various points which 

 have been discussed, but the difficulty in many cases is to be sure of 

 the sequence. Where there are scattered deposits, with anisidophorous 

 faunas, in contiguous localities of the same area, they cannot be of the 

 same date, though they occupy the same relative positions. But there 

 may be, to hand, little or no clue to sequence. For instance, species of 

 Macrocephalitidas occur in Cornbrash limestone, Kellaways Clay and 

 Kellaways Rock. Where these are found super-imposed in one small 

 area, the sequence of their contained species is known — so far as the 

 three rocks are concerned ; but the sequence of species in each rock 

 may not be known. Where these rocks occur in widely-separated areas, 

 their sequence can be only surmised — for the supposed Kellaways Clay 

 may be a local argillaceous contemporary of the Cornbrash, while the 

 local Kellaways Rock, instead of being later than the Kellaways Clay 

 elsewhere, may be earlier or synchronous. The sequence, then, of 

 species of Macrocephalitidse from widely-scattered localities .along the 

 Bathian-Callovian junction can only be a matter for surmise — it cannot 

 be stated from their matrices — not till all forms have been thoroughly 

 worked out and definite local super-positions of strata with distinct 

 forms have been ascertained. 



