1922 CHROXOLOGY 21 



Dec. 



This mutability of lacustrine, estuarine and marine conditions 

 presents some interesting problems. For instance, where, at the com- 

 mencement of the Ammonitoidic Period, did the Jureuropean 

 land-locked sea have connexion with the ocean ? It may be suggested 

 that it was in south-eastern Europe, and yet that the irruption which 

 opened up the Jureuropean Sea came from the west. At any rate, 

 two gateways for this sea were, later, developed on the west — that of 

 Bordeaux, which was to the south of Juroceltia, and that of Ireland- 

 Hebrides, which was to the north of it. • Differences in the distribution 

 of species of PsUoceras should give evidence as to whether these two 

 gatewaj's were breached separately or simultaneously, just as the absence 

 from Wurtemberg of the geologically-earhest forms of PsUoceras — those 

 of the P. planorbis-type — seems to supply one piece of evidence, not only 

 against the irruption being from the east, but in favour of a temporary 

 closing up of any connexion that may have formerly existed in that 

 direction. 



There is, however, another piece of evidence — the difference in the 

 Caloceratan fauna of Wurtemberg and the North-Eastem Alps. But 

 this raises the whole question provoked by a consideration of Table I — 

 why is a difference of fauna considered, in the one case, to imply absence 

 of strata ^^ithout difference of province, and, in another case, difference 

 of province ^^ithout necessarily absence of strata ? 



The faunal differences are not of the same value. In the one case 

 there are, say, two localities. A, B, which may be hundreds of miles 

 apart. In the case of A the faunal (Ammonite) sequence may be 

 represented by the letters a, c, d ; but in the case of B the faunal sequence 

 stands as a, b, c, d. In the respective faunas a, c, d, the identity of 

 species is so great as to warrant the assumption that there was free 

 inter-communication between the two localities. The difference between 

 them hes in the absence of the fauna b from the locaUty A, but its presence 

 in the locality B. It is, then, reasonable to assume that such absence 

 from the locality A was due to penecontemporaneous removal of the 

 deposit which contained fauna b — an assumption which can generally be 

 strengthened by investigation of localities intermediate between A and 

 B : some localities will show presence of fauna b, others not only absence 

 of fauna b, but absence of any strata between the beds containing 

 a and c. 



In regard to two localities, B and C, which may be only a few score 

 miles apart, the faunal difference may be expressed, in the one case, as 

 before, a, b, c, d ; but, in the other case, as ai, bi, ci, di : the species 

 in the faunas are not identical — the species are morphic equivalents, 

 they are similar in general facies at each respective faunal horizon. 

 In such case it is reasonable to assume that the respective faunas belonged 

 to separated geogiaphical provinces, that there was some barrier which 

 prevented all communication between the respective areas; because, if 

 there was any communication, there would be a minghng of a, b, c, d 

 and ai, bi, ci, di, since the distance is quite short ; seeing that there 

 are cases of identity of species pointing to free communication when the 

 distance is far greater. And another piece of evidence supports the 

 assumption of distinct provinces — that in the case of C there is greater 

 faunal (Ammonite) richness of species than in B. 



If A be taken to represent South England, B, Wurtemberg, and C, 

 the Xorth-Eastern Alps, then the above reasons show why Dorset and 

 Wurtemberg may be considered to be parts of one province, while the 

 North-Eastem Alps may be supposed to be separated from that pro\nnce 



