230 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [Jan. 24, 



geologists how close an agreement existed between the marine fauna 

 of the Suffolk Crag and that of Antwerp. At the same time it was 

 evident from the work of M. Nyst * that there were many forms 

 met with in one area which were apparently wanting in the other, 

 and rendering it a matter of some interest in comparative geology 

 that such a difference within such narrow limits should be accounted 

 for ; this difficulty has long been a point of special interest in Ter- 

 tiary geology. The solution has been heretofore attempted by 

 various applications of the percentage test, as by the proportion of 

 recent forms to such as are unknown as living, or again by the pro- 

 portion which a given parcel of shells might contain, either of Arctic 

 or of more Southern forms. There has been so little agreement in 

 the results thus arrived at that some fresh solution should be 

 attempted. Percentage results, from their very nature, must ever 

 be fluctuating ; the process when first proposed seemed specious, in- 

 asmuch as it had the appearance of possessing arithmetical accuracy, 

 but in reality it is the very reverse of this, in consequence of the 

 uncertainty of all the elements of the calculation and of their nega- 

 tive character. 



M. Dumont proposed a twofold division for the Tertiary or Kaino- 

 zoic series of Antwerp, an upper or Scaldesien, a lower or Diestien. 

 Sir Charles Ly ell's account of the Antwerp Crag was prepared under 

 disadvantageous circumstances. At that time a few detached exca- 

 vations afforded the only attainable stratigraphical information, and 

 hence it became necessary to give the fossils collected at each loca- 

 lity in separate lists, thus producing the impression that the Crag- 

 formation there was both complicated and of considerable vertical 

 thickness, but such is not the case. 



At the time of our visit an examination of the Antwerp beds was 

 extremely easy by means of two long artificial sections, one passing 

 along the main ditch of the new " enceinte," 14,000 metres in 

 length, the other along the ditch of the detached Forts, with a 

 length of 17,000 metres; thus giving 31,000 metres of continuous 

 section. These sections formed the subject of a short but very 

 useful memoir by M. Dejardin, Capt. de Genie, one of the officers 

 engaged on the workf. 



In the excellent Memoir by Sir Charles LyeU J, which has done so 

 much to make English geologists acquainted with the relations of 

 the various subdivisions of the Nummulitic and Tertiary formations 

 of Belgium to those of this country, the Tertiary series is arranged 

 as follows : — 



§ 3. Antwerp Crag. 



1. Yellow Crag-UpP-Crag {s°S£g% ni.l ^,,^^^ ^^. 



2. Crag gris — Middle Crag — tab. iv. j -^ ' 



3. Crag noir — Lower Crag — tab. v. J 



§ 4. Sands and Iron Sandstone of Diest — " Systeme Diestien " of Dumont. 

 § 5. Bolderberg Sands — " Systeme Bolderien." 



* Coquilles Tertiaires de la Belgique. 



t Bull, de I'Aoad. E. des Sc. de Belgique, t. xiii. p. 470. 



X Quart. Journ. Geol. Soc. vol. viii. p. 277, 1852. 



