ROBERTSON ON THE WEALDEN BEDS OF BRORA. 117 



obtained, the included fossils would probably be associated in this 

 way. Commencing at the fluviatile extremity of the estuary, we 

 should find, in the first place, strata with terrestrial and freshwater 

 exuviae only. Proceeding seaward, these would gradually be re- 

 placed by true estuary deposits, containing species belonging to 

 marine genera, in addition to the remains of the preceding divi- 

 sion. There would next be a transition to marine beds, enclosing, 

 however, along with the beings proper to the region, a considerable 

 amount of such terrestrial organic remains as had been carried 

 out to sea ; and we should have, lastly, an assemblage of deep sea 

 fossils, with occasional fragments of wood, or other buoyant terres- 

 trial substances. Certain fishes, like the modern Salmonidse and 

 Sturionidae, and also, in all probability (if we may judge from 

 their usual associates), various species of Lepidotus and other 

 allied genera, whose migratory habits carried them alternately into 

 fresh and salt water, would be found to prevail more or less through- 

 out the whole of the beds. Others again, so constituted as to be 

 able to live in the ocean and in brackish water, but not in perfectly 

 fresh water, would occur in the last three, although not in the first, 

 of the above divisions. We may conjecture, on the ground of asso- 

 ciation above suggested, that the latter class was formerly repre- 

 sented by the Cestraciontidse and Hybodontidae. 



Of a succession of deposits such as I have supposed, formed 

 during the Jurassic period, the first and second divisions would be 

 referable to the Wealden ; tlie third would closely resemble the 

 Stonesfield slate, Caen limestone, &c.; and the fourth would be 

 identical with one of the marine members of the system. Had the 

 land been in a state of upheaval during the accumulation of the 

 various beds, the first and second classes would overlie the others ; 

 .but were it, on the other hand, subsiding, the reverse of this would 

 take place. It seems consequently not unreasonable to infer, that 

 Wealden beds may yet be found, in contact with the Stonesfield 

 slate and other similar deposits, their relative position depending 

 upon the upward or downward motion of the land at the time of 

 their accumulation. 



The Neocomian question has recently directed so much atten- 

 tion to the junction of the Jurassic and Cretaceous systems, in va- 

 rious countries where the Wealden above the Portland stone does 

 not occur, that, from the universal absence in such localities of 

 any bed not referable either to the last-mentioned rock or the lower 

 greensand group, the addition of other members to this part of the 

 geological series can scarcely be looked for. Assuming this to be 

 the case, I shall proceed to inquire, whether the Wealden of the 

 north of Germany*, south-east of England, and north of France, be 



* From the brief account given by M. v. Romer of the Wealden strata of this re- 

 gion (Versteinertangen des Nord-Deutschen oolithen Gebirges ; Nachtrag, s. 6 et 

 seq.),l suspect that some of the beds are below the Portland stone : thus M. v. Romer 

 mentions that at Klein Siintel the Weald clay is absent, but there are 450 feet of 

 sandstone with seven seams of coal, resting on clay and shale 240 feet thick, " of 

 which at least the lower part belongs to the inferior oolite." 



