500 Geological Society, 



which a permanent alteration in the inhabiting medium may work 

 in the form and size of a shell or coral. 



Of the importance of organic remains in identifying districts less 

 widely separated, the two following instances were noticed. In 

 M. Dumont's work on the geology of the province of Liege, pub- 

 lished in 1832, and justly valued for unravelling the structure of a 

 most intricate country, the strata immediately beneath the mountain 

 limestone are divided into three systems, but without any definite 

 comparison with the formations which underlie that deposit in 

 England. At the meeting of the Geological Society of France, at 

 Mezieres, in September, 1835, Dr. Buckland proposed the following 

 first comparison between the systems of M. Dumont and the sub- 

 divisions of the Silurian system established by Mr. Murchison : — 



Systeme calcareux superieur Mountain limestone. 



(Old red sandstone wanting.) 

 Syst. quartzo-schisteux superieur. . The Ludlow rocks. 

 Syst. calcareux inferieur The Dudley and Plymouth lime- 

 stone. 

 Syst. quartzo-schisteux inferieur. . The Caradoc sandstone. 



(Builth and Llandeilo flags wanting.) 

 Terrain Ardoisier. 



This comparison was principally founded on the resemblance 

 of the corals with those obtained at Dudley and Wenlock. M. 

 Constant Prevost pointed out the resemblance of the calcaire bleu 

 of the systeme calcareux inferieur of M. Dumont with the Plymouth 

 limestone, and of the marble of Heer, subordinate to the systeme 

 quartzo-schisteux superieur, with the limestones of Babacombe. 

 Mr. Greenough, however, doubted the identity of the Plymouth 

 and Dudley limestones, and he stated that he had remarked the 

 total absence of the Dudley Trilobites in the systeme calcareux 

 inferieur. During the Mezieres meeting, Dr. Buckland identified 

 certain beds beneath the mountain limestone near Namur, Di- 

 nant, and Huy, and at Engis, with the old red sandstone * ; and 

 at an ordinary meeting of the Geological Society of France, in 

 December, 1837, M. Rozet repeated his belief, that the old red 

 sandstone is well developed between Dinant and Namur ; and M. 

 Constant Prevost stated, that he had also during the Mezieres 

 meeting, determined its existence in those districts. In 1838, 

 M. Dumont visited England for the purpose of examining the 

 Silurian region ; and on his return to Belgium, he laid before 

 the Royal Academy of Bruxelles a table, differing from that of 

 Dr. Buckland only in drawing more closely the terms of com- 

 parison, and in identifying the two upper divisions of the Terrain 

 Ardoisier with the Cambrian system. He stated also, in a report 

 which accompanied the table, that the old red sandstone was 

 most probably wanting in Belgium, or, if it exist, that it must 



* In the " Outlines of England and Wales " (1822), the Rev.W. D. Cony- 

 beare places all the Belgian beds between the carboniferous limestone and 

 the transition slates in the old red sandstone. — Note, 468. 



