ROBERTSON ON THE WEALDEN BEDS OF BRORA 119 



be only one natural process capable of producing the simultaneous 

 extinction of the marine species of any region, viz. the sudden 

 elevation and desiccation of the bottom of the ocean in which they 

 lived. 



That at the close of the Jurassic period an elevation took place, 

 resulting in the conversion into land of considerable portions of the 

 bed of the ocean in which the Portland stone was deposited, is un- 

 equivocally proved by the occurrence of " dirt-beds" in the south- 

 east of England and north of France, as well as by the coal-seams 

 of the same age in Hanover. From the few localities in which the 

 equivalents of the Portland stone have been recognized, it may be 

 inferred that, at the time of its deposition, by much the greater por- 

 tion of Europe had already been raised above the waters of the 

 ocean ; and there is, therefore, nothing unreasonable in the conjec- 

 ture, that a moderate change of level afterwards sufficed to extend 

 the desiccated area, so as to include, or even exceed, the limits of 

 the territory now exposed to geological investigation. The upheaval 

 must have taken place suddenly, as is shown by the distinct plane 

 of separation, and the absence of alternations observable at the junc- 

 tion of the Portland stone and Purbeck beds. Admitting then that 

 the method suggested above is that by which the extinction of ma- 

 rine animals over a certain area is accomplished, we have, in the 

 changes alluded to in the preceding sentences, a cause for the dis- 

 appearance of the species of the Portland stone from the European 

 region. 



When the alterations in the distribution of land and water, in the 

 depth of the ocean and in the nature of its deposits, as well as other 

 mutations affecting the characteristics of species (all necessarily 

 consequent on the conversion of a portion of the submarine area into 

 a terrestrial condition), are considered, it is not difficult to imagine 

 that the inhabitants of the waters which washed the shores of the 

 new continent would differ as much from their predecessors as the 

 fossils of the lower greensand do from those of the Portland stone. 

 Under such circumstances I conceive the first inhabitants of the 

 lower greensand to have made their appearance. 



Transition beds must exist in which individuals of both the old 

 and new races are interred, but they could only have been formed 

 on those parts of the ocean bed which remained submerged after the 

 elevation so often mentioned took place ; and as none have been 

 found, notwithstanding the most diligent search in all the better- 

 known countries of Europe, it must be presumed, as already hinted, 

 that the continent extended farther to the westward than it does at 

 present, and that they lie buried under the waters of the Atlantic. 



As the "dirt beds" and coal prove that elevation of the European 

 region followed the deposition of the Portland stone, so do the lower 

 greensand strata, which rest upon the Wealden deposits, show that, 

 before they could have assumed the position in which they are now 

 found, subsidence must have happened. The Iguanodon of Maid- 

 stone, and the Lonchopteris Mantelli of the Isle of Wight, further 



