

CHAP. IX.] JURASSIC PERIOD. 161 



perhaps is pushing the inference too far, and the conditions 

 of the Purbeck basin seem to me more correctly described 

 as those of a silted-up gulf or bay, portions of which be- 

 came freshwater lakes or meres, such as formerly existed 

 in the Fenland of Norfolk and Cambridge. It is true that 

 the conditions of sedimentation in the Purbeck bay were 

 evidently different from those in the great bay of the Fen- 

 land ; the latter has been filled up chiefly by tidal silt, and 

 the areas of freshwater deposits are very small, while in 

 the former the lacustrine areas were large, and silt-bearing 

 currents were absent, so that even the marine deposits 

 were calcareous. Still, there is much analogy between the 

 cases, and I therefore agree with the wording of Mr. 

 Meyer's conclusion that it was " by the co-existence within 

 a comparatively wide area of a fauna suited respectively 

 to freshwater and brackish-water conditions, and by the 

 interchange of such conditions over portions of the same 

 area, supplemented by the occasional intrusion of the 

 ocean, that I would account for the alternation of fresh- 

 water, marine, and brackish-water fossils in the Purbeck 

 strata ; for neither the conditions of their accumulation 

 nor the life-conditions of their fauna appear to be suffi- 

 ciently in accordance with an estuarine position " (loc. cit., 

 p. 246). 



On this view, too, the stratigraphical relations of the so- 

 called Purbeck beds of Oxford and Bucks admit of easy 

 explanation ; for if our view of Portlandian geography is 

 correct, it is exactly in this Midland district where we 

 should expect to meet with evidence of the early prevalence 

 of such conditions, and to find freshwater strata of an 

 earlier date than that of the Dorsetshire Purbecks. There 

 we find beds with Portlandian fossils passing up into 

 purely lacustrine strata, which were evidently deposited 

 in the quiet waters of a lake that was never invaded by 

 strong currents either of fresh or salt water. We may 



M 



