ROBERTSON ON THE WEALDEN BEDS OF BRORA. 123 



tween the Jurassic system and Wealden, the total absence throughout 

 the latter series of a single marine remain common to it and the 

 lower greensand is equally conclusive against the supposition that 

 the strata in question were formed in an estuary communicating 

 with the cretaceous ocean. 



Enough has now been said to show cause for the belief, that, 

 at all events, the greater part of the Wealden beds above the Port- 

 land stone were deposited contemporaneously with a portion of the 

 lower greensand ; and also to indicate the means by which the ter- 

 restrial and freshwater, and even some of the marine inhabitants of 

 a period may continue to exist long after the extinction of its truly 

 oceanic tribes. 



It may appear to some geologists, that the word ' Wealden,' 

 being no longer exclusively applicable to those interesting deposits 

 which occasionally intervene between the Portland stone and lower 

 greensand, ought to be suppressed ; but until the science is so far 

 advanced as to afford criteria for distinguishing the strata of deltas, 

 estuaries and inland seas from each other, it is convenient to have a 

 collective expression for all such beds belonging to the Jurassic 

 system. 



In conclusion, I have only to observe, that the hypothesis advo- 

 cated in the latter part of this paper may, I think, with slight modi- 

 fications, be rendered explanatory of other geological phaenomena 

 than that to which it particularly refers *. 



Appendix. 



[Dated 2nd December, 1846.] 



After an anxious consultation of all the authorities within my 

 reach, I had failed, when the preceding pages were transmitted to 

 the Society, in obtaining sufficient information with respect to the 

 Caspian Sea, to enable me fully to carry out the theory proposed in 

 explanation of the phaenomena of the Wealden series above the 

 Portland stone, and of the relations of these strata to the Jurassic 

 and Cretaceous systems ; but having subsequently learned, through 

 the kindness of one of its distinguished authors. Sir R. Murchison, 

 that the requisite details were to be met with in ' The Geology of 

 Russia and the Ural Mountains,' &c., I was much gratified, on pro- 

 curing the volumes, to find that the ample account therein given, of 

 the comparatively modern changes which have happened in the Aralo- 

 Caspian region, tended to support my hypothesis. The passages 

 having reference to the subject, all of which occur in the first volume 

 of the work above mentioned, will be found in the succeeding para- 



* The description of the new species from the beds of Brora is postponed 

 until more perfect specimens are obtained. The best of those already met with have 

 been sent to the Geological Society, in order that any one wishing to compare them 

 with the similar remains of other localities may have an opportunity of doing so. 



