DEVONIAN AND OLD RED SANDSTONE SYSTEM 215 



UP I" Red and grey marbles with Goniatites. 

 1000 f 'tl Unfossiliferous sandstones (500 feet). 



I Limestone of Candas with Spirtfcr Verneuilli. 



I Sandstones with Qosseletia dcronica (150 feet). 



Middle, I Limestones of Moniello with Calceola sandalina and Cysti- 

 1000 feet | phyllum vesiculasu ///. 



\. Limestones of Arnao with Spirifcr cultrijugatus. 



'Limestones of Ferrones with Spiri/er paradoxus, Ithynchontlla 



Lower, 



about 



1800 feet 



Orbignyana, and many corals. 

 Calcareous slates and limestones with Spirifer hyattricus, 



Jihynch. pila, and Homalonotus. 

 Furada grits, red and green grits with green and purple slates 



over 600 feet thick. 



The lowest beds are comparable with the Plougastel grits and 

 slates, and the overlying limestones are calcareous equivalents of 

 the Gahard and Faou sandstones. The limestones of the Middle 

 Devonian correspond with the beds which are known as Eifelien 

 in the Ardennes, and it is curious to find the upper part of the 

 series, which is usually calcareous, here represented by sandstones. 

 In the Upper Devonian again there is a still thicker band of 

 sandstones, proving that the deposition of limestone was temporarily 

 interrupted by the invasion of a sand-bearing current, and that 

 continental land was still not far away. 



3. The Ardennes 



In the Ardennes Devonian rocks occupy the greater part of an 

 area lying partly in France and partly in Belgium, which is over 

 100 miles in length from west to east, and has a maximum width 

 of about 50 miles (see map, Fig. 70). The area really consists of 

 two basins, one broad and regular (basin of Dinant), the other deep, 

 narrow, and isoclinal (basin of Namur), which are brought into 

 apposition by a reverse fault (see Fig. 71). The throw or displace- 

 ment of this fault amounts to many thousand feet, and the strata of 

 the two basins must originally have been separated by an interval 

 of several miles. That this was so is proved by the fact that whi li- 

 the southern basin, that of Dinant, contains a very thick repre- 

 sentative of the whole Lower Devonian Series (average about 

 8000 feet), this is entirely absent in the basin of Namu-. 18 



Lower Devonian. On the south side of the basin of Dinant 

 the Lower Series has a total thickness of about 10,000 feet At 

 the base, resting unconformably on Cambrian, is a conglomerate 

 overlain by coarse felspathic sandstone or arkose ; this is succeeded 

 by a series of hard grits and slates, generally of green or red tints, 

 but containing very few fossils. A fossiliferous band at Montrepuits 



