parti] THE SHALES-WITH-' BEEF.' 99 



surfaces was that the limestones represent precipitates of calcite- 

 mud derived by the degradation of Carboniferous Limestone areas. 

 The exact character of each precipitate varied according to slight 

 changes in the salinity of the waters. Similar results could be 

 obtained by the slow precipitation of clay-suspensions under the 

 influence of electrolytes. The speaker had not yet had an oppor- 

 tunity of trying similar experiments with calcite-suspensions, but 

 he had little doubt that under suitable conditions similar effects 

 could be brought about. 



Miss C. A. Raisin asked whether, from the study of the inter- 

 esting cone-in-cone structure in Jurassic strata, Mr. Richardson 

 considered there was any evidence that contraction due to the 

 drying of the material had acted as a partial cause, in conjunction 

 with crystallization and the other forces indicated. 



Dr. Lang thanked the Fellows for their reception of the paper, 

 and, in reply to Dr. Bather, mentioned that he had collected many 

 lamellibranchs and other fossils from the Shales-with-' Beef,' 

 but had not found a specialist to name any, except the ammonites; 

 he had, therefore, recorded in his paper but few fossils besides 

 ammonites. As to faunal repetition, if we could trace our re- 

 stricted horizons laterally, we should undoubtedly discover the 

 intermediate histories of- the temporarily eclipsed genera. Two 

 possibilities were suggested : — (1) The reappearing form might 

 not really be closely related to the vanished form — the supposed 

 genus might be polyphyletic. (2) The discontinuous succession 

 of the forms of a given genus in one locality might be the result 

 of a periodic outpouring of allied forms from a more or less distant 

 centre. If we could find such a centre, we should see that there 

 the vertical distribution of the genus in question was continuous. 



Mr. W. A. Richardson said that it was very difficult, in the 

 present state of knowledge, to fix the exact date of nodular forma- 

 tion ; but it occurred at Charmouth before the crushing of the 

 ammonites, and probably while the sediment was still comparatively 

 loose. In reply to Prof. Cox it might be pointed out that the 

 same peculiar rhythm occurs in many nodular deposits (such as 

 flint and gypsum), and it was the difficulty of finding any external 

 cause varying in this way that supplied the chief reason for resort- 

 ing to an ' inner,' that was, a physical-chemical cause. Regarding 

 Mr. Flymen's query, the bands were formed simultaneously, only 

 in the sense that they were the result of one continuous process of 

 precipitation. There would not be much scope for contraction as 

 a cause of cone-in-cone, as suggested by Miss Raisin, for the 

 material was deposited initially and continuously in crystalline 

 form. 



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