236 EEV. J. F. BLAKE ON THE POETLAND EOCKS OF ENGLAND. 



the Lower Portlandian (zone of Ammonites gigas) in the south of 

 England. 



Prof. MoEEis bore witness to the accuracy of the observations of 

 the author. He referred to the bed of phosphatic nodules which, 

 in the Aylesbury area, overlies the Kimmeridge, and contains a 

 number of peculiar fossils. This bed has been traced for some 

 distance. He regarded Cytlierea rugosa as a form of Cyrena and 

 indicating the first incoming of estuarine conditions. At Hartwell 

 the transition from marine to freshwater conditions is very well 

 marked. 



Prof. Seeley thought that part of the beds in the central districts, 

 now regarded as ISTeocomian, arc really, as supposed by William 

 Smith, of Portlandian age. He regarded the Portlandian as essen- 

 tially a sandy series, graduating down into the Kimmeridge Clay 

 below and the ISTeocomian above, and considered that the Portland 

 Limestones were mere accidents due to differences of condition. 



Prof. T. P. Jones had not succeeded in finding fossils in the black 

 clays of Swindon which would enable us to decide whether they are 

 of freshwater or of marine origin. He noticed the occurrence of 

 worn septaria on the top of the side of the quarry ; and stated that 

 he had found fossil wood, like that of the Purbeck beds at Portland, 

 in another bed near the top. He remarked that a chalky bed in the 

 Portland beds at Upway contains Cretaceous Entomostraca. 



Prof. Ramsay informed the author that the block in the Museum 

 of Practical Geology, referred to in the paper as being Portlandian 

 in the lower part and Purbeck in the upper, contained Cyprids in 

 the higher part of it. He agreed with the author as to the phy- 

 sical geography of the district during the Portlandian period. 



Mr. Whitaker stated that at the time when the Geological 

 Survey of Buckinghamshire and Oxfordshire was made it was found 

 very difiicult to separate the Portland Limestone from the Portland, 

 Sand. 



Prof. Waltee Keeping referred to the existence of Portlandian 

 forms in the Potton and similar beds in the Midland Counties. He 

 thought that these were derived from Portlandian strata, which 

 must have been originally spread over wider areas. He thought 

 that the black pebbles in the Portlandian and Potton beds are de- 

 rived from Carboniferous rocks forming part of the great Palaeozoic 

 ridge. 



The AuTHOE did not agree with Mr. Hudleston as to the limits 

 of the Trigonia gihbosa. He did not regard the "roach" as a 

 continuous deposit, since in diff'erent sections it occurs at difi'erent 

 horizons. The zone of Ammonites gigcis is certainly not to be dif- 

 ferentiated as a distinct series in the Kimmeridge section. Possibly 

 a representative of the Portland may exist in Lincolnshire. ' He did 

 not regard Entomostraca as being always good characteristic fossils. 



