STRATIGRAPHICAL GEOLOGY. 529 



of the Pliocene or English " Crag." The stratigraphy of the 

 Tertiary deposits so ably described by Brongniart was further 

 investigated by several eminent French geologists, a very 

 suggestive paper on differences of facies being contributed 

 in 1838 by C. Prevost (cf. p. 503). Hebert in 1848 threw 

 new light upon many of the stratigraphical features, especially 

 the structural relations at the margins of the basin. 



In England, Joseph Prestwich l had commenced his studies 

 of the two Tertiary basins of Hampshire and London in the 

 year 1 846. He contributed a series of memoirs to the Journal of 

 the Geological Society^ all of which display remarkable scientific 

 judgment and accuracy of observation. Prestwich demon- 

 strated for the first time the presence of Thanet Sands as 

 a well-defined zone below the London Clay, and showed that 

 the latter was not the equivalent of the Bracklesham and Barton 

 strata, nor of the " Coarse limestone " of the Paris basin, but 

 belonged to a deeper horizon. In a memoir published in 1855, 

 Prestwich made an attempt to compare the older Tertiary 

 groups of England with those of the Paris basin and Belgium, 

 relying upon the results of D'Archiac and Dumont for his data 

 regarding the Continental deposits. Both these authors had 

 previously drawn up synchronous tables for the English and 

 Continental developments, but the subsequent researches of 

 Prestwich enabled him to make certain alterations from the 

 English standpoint. 



The only foreign equivalent which Prestwich could find for 

 the Thanet Sands was the lower part of the Belgian Landenien 

 (Heersien) ; in the Paris basin he regarded the lower glau- 

 conitic marine sands (Sables de Bracheux), the plastic clay, 

 the lignite and the conglomerate of Meudon as equivalent of 

 his Woolwich Series ; true London Clay seemed absent in the 

 Paris basin, but was represented in Belgium by the lower 

 Ypresien of Dumont. The Lower Bracklesham or Bagshot 

 strata were represented by the sands of Soissons, Cuise, Aizy, 

 and Laon in the Paris basin, as well as by the upper part 



1 Sir Joseph Prestwich, born 1812 at Pensbury near London, was educated 

 partly in England, partly in Paris, and after the completion of his studies at 

 University College in London, he entered his father's business, from which 

 he only retired in 1872. All his leisure was devoted to geological re- 

 searches, and in 1874 he succeeded J. Phillips as Professor of Geology in 

 Oxford. In 1888 he was President of the P'ourth International Geological 

 Congress in London; he died on the 23rd June 1896. 



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