120 MESSRS. JUKES-BROWNE AND SCA.NE8 ON THE [Feb. I9OI, 



VII, CONCLtrSIONS. 



The sections described in the foregoing pages show that in this part 

 of Wiltshire both the Selbornian and the Cenomanian stages 

 are very fully represented. It is convenient to use these terms, 

 because they denote definite divisions or stages in a palaeontological 

 scheme of classification ; whereas the terms which have hitherto been 

 in use in this country (namely, Gault, Upper Greensand, and Lower 

 Chalk) denote merely local lithological facies of one or the other stage. 

 The relations of the beds which constitute the Selbornian have been 

 discussed elsewhere,^ and it has also been shown that the Chloritic 

 Marl (or sub-zone of Stauronema Carteri) belongs to the Cenomanian 

 stage,^ which in England has generally, but not always, a chalky 

 facies. 



In some districts, notably in Kent and Dorset, there is a marked 

 plane of separation between the Selbornian and the Cenomanian 

 stages, so that there is no difficulty in those counties in deciding 

 which beds belong to each. Elsewhere, however, as in Southern 

 Wiltshire and in the Isle of Wight, there, is more difficulty because 

 a set of passage-beds come into the sequence, which do not exist 

 either in Kent or in Dorset. 



These passage-beds have evidently been formed during an epoch 

 of transition, when the physical conditions which had prevailed 

 during the Selbornian age were being changed and those of the 

 Cenomanian age were being ushered in. It was a time when parts 

 of the sea-floor were swept by strong currents, for these passage- 

 beds almost always exhibit clear evidence of current-action, such as 

 uneven floors, piped surfaces, phosphatic concretions, and water- 

 worn stones ; while the absence of such passage-beds in Kent and 

 Dorset is evidently due to the action of still stronger currents. 



It is obvious that deposition may have been going on in one place 

 while erosion was in progress not far away ; and consequently it is 

 improbable that a break or clear plane of demarcation, observable 

 at one locality, should occur exactly on the same geological horizon 

 as that seen at another locality. 



In considering any single exposure, or those within a small area 

 where fossils are abundant, the local evidence must of course be 

 reviewed on its own merits. When, however, we come to consider 

 the relations of one formation to another throughout a large area 

 or region, such as the whole of Southern England, and have to decide 

 on the most convenient plane of demarcation between them, then we 

 are bound to take into account all the facts of the case, and the plane 

 decided upon should be that which harmonizes best with all these 

 facts, both stratigraphical and ontological. In other words, a line 

 which seems convenient at one locality, where perhaps the succession 

 is especially complete, may or may not be fittingly applicable to a 



1 Mem. Geol. Surv. * Cretaceous Eocks of Britain' vol. i (1900) * Gault & 

 Upper Greensand of England * pp. 3, 30. 



=» Quart. Journ. Geol. Soc. vol. lil (1896) pp. 171, 172. 



