1872.] MEYER WEALDEN AND " rUNFIELD FORMATION." 245 



shire Purbecks, read before the British Association in 1850, that a 

 change from freshwater to marine conditions took place suddenly 

 on at least two occasions ; and it is evident that such changes took 

 place at once over the greater part, if not the whole, of the Purbeck 

 area. It is on these occasions, and on these only, that true marine 

 fossils (Pec^eji, Avicula, Thracia, Ilemicidaris) maAie their appearance 

 in the Purbeck strata. It is certainly a fact that there is no real 

 intermingling of freshwater and marine genera in the same stratiim ; 

 the abrupt change from freshwater to marine conditions is again and 

 again succeeded by a more gradual change from brackish to fresh- 

 water. Now it seems to me that such sudden change from fresh- 

 water to marine conditions, and the gradual return to freshwater, 

 accords well with the idea of the intrusion of the ocean into a lake 

 or basin of fresh water, and does not accord with the results usually 

 due to tidal action ; for it is evident that the freshwater fauna of the 

 Purbecks is lacustrine rather than fluviatile, while the brackish- 

 water sj)ecies are such as might well have lived in waters not open 

 to the sea. 



Judged therefore by its fauna, no less than on stratigraphical 

 evidence, the condition of the Purbeck basin was probably from its 

 commencement rather that of a lake, a series of lagoons, or even of 

 an inland sea, than of an estuary in the ordinary meaning of the 

 word. Its waters were mostly shallow,and possibly always brackish 

 some portion of its area, as in those more distantly removed from 

 the influx of rivers, becoming locally almost salt at times from actual 

 evaporation. 



That its waters were for the most part shallow there is abundant 

 evidence in many portions of the Purbeck strata ; and taking into 

 account the j^robable climatal conditions of England during the 

 Purbeck and Wealden periods, as shown by its flora, the occasional 

 drying up, or even evaporation to saltness, of portions of such a lake 

 or inland sea can hardly be thought an unreasonable supposition. 



I admit that the evidence as to the effect of evajporation in pro- 

 ducing saline conditions in these strata is not clear ; but that some 

 such action was in progress during the accumulation of the Purbeck 

 strata I shall attempt to prove by specimens from Durlstone Bay. 



These specimens, which Professor Morris tells me are both rare 

 and curious, are cubical siliceous pseudomorphs of crystals of salt. 

 The stratum in which they occur is crowded with fossils of purely 

 freshwater genera ( ZJnio, Physa, Valvata, Paludina, Plaaorbis, besides 

 innumerable valves of Cypris and seed-vessels of Cliarce), the pre- 

 ceding and succeeding strata containing also the same fossils with- 

 out any intermixture of marine species. But the importance of the 

 evidence afforded by the accidental preservation of salt crystals in 

 such a position consists in the fact that the surface of the stratum 

 which contains them is traversed by sun-cracks, proving, as it seems 

 to me, that the saline conditions of the moment were quite possibly 

 the result of rapid evaporation. 



It is therefore by the coexistence within a comparatively wide area 

 of a fauna suited respectively to freshwater and brackish-water con- 



