THE CARBONIFEROUS SYSTEM 311 



40 feet thick, and thus separates the Visuan shales (see p. 279) from 

 the overlying Coal-measures. These latter have been divided by 

 Zeiller 9 into the following zones : 



I. /"Me of the " charbons flnus " (gas-coals) with Pecopteris nervosa, P. 

 '/'n'ti/ii, X'-nropteris rarinervis, Sphenvpteris obtusiloba, and Sigil- 

 laria tcsselata. 



0. /one of " charbons gras," with Sphenopteris nummularia, Neurop- 



teris gigantea, Aletfwpteris Serli, and Sigillaria polypltea. 

 2. Zone of "charbons demigras " Sphenopteris trifoliata, Lonchopteris 

 Bricei, Aleth. Darreuxi, and Sigillaria scutellata. 



1. Zone of "charbons maigres " Neuropteris Schlehani, Alethapteris 



lochitica, Sigillaria elegans, and some Lepidodendra. 



It will be noticed that the succession of coals and of plant-zones 

 is similar to that of Westphalia. The total thickness of this series 

 near Boussu west of Mons is about 2400 metres ( = 8000 feet), and 

 near Douai it is estimated to be more than 2500 metres. 



In the Boulonnais, near Leulingham, Locquinghem, and Ferques, 

 Coal-measures are being worked under overthrust masses of Car- 

 boniferous limestone ; and another trough appears to come in to 

 the north of these overthrusts, part of it lying below Dover where 

 Coal-measures were reached by boring at a depth of 1100 feet from 

 the surface. The same series of trouglis and faulted blocks is 

 probably continued westward under the south of England till 

 it emerges again in the coalfields of Bristol and Somerset 



3. Sarrebruck Area 



Just as the Belgian coalfields lie in a synclinal trough on the 

 northern flank of the broad tract of Devonian rocks which stretches 

 across the Ardennes and the Rhine Valley, so also we find another 

 syncline on its southern flank. The exposed portion of this 

 syncline is known as the Sarrebruck basin, but it is very likely to 

 have a prolongation westward beneath Metz, though probably at 

 too great a depth to be attainable (see Fig. 103). 



The deposits in this southern basin, however, differ much from 

 those in the northern, for not only is there an entire absence of 

 the Dinantian Series, but the greater part of the Westphalian is 

 also wanting, and the lowest beds in the basin must rest uncon- 

 formably upon the Ix)wer Devonian or still older rocks. The 

 bottom of the basin in its deepest part has not yet been reached, 

 but the lowest beds traversed correspond with the highest part of 

 the Belgian Series (the charbons fl^nus) and are covered by beds 

 which resemble our Upper Coal-measures, while these latter are 

 succeeded by the still higher Stephanian Series. The Westphalian 

 part of the succession is as follows : 



