DEVONIAN AND OLD RED SANDSTONE SYSTEM 213 



west, Brittany, Normandy, and Anjou, and in the north-east in 

 the region of the Ardennes. It will be convenient to begin with 

 Brittany, which is the nearest to our own Devonian area. The 

 western facies of the system is found in the basin of Finisterre, 

 which extends from the Rade de Brest to Lanfains and Uzel 

 north of Loudeac. For our knowledge of this area we are 

 chiefly indebted to the labours of Professor C. Barrois, who has 

 described the succession as consisting of three natural lithological 

 divisions corresponding with differing bathymetrical conditions of 

 deposition, i.e. shallow, deeper, and deep. 10 The lowest and larger 

 part is arenaceous with a thickness of about 3000 feet ; the middle 

 part consists of gritty slates and mudstones, with lenticular beds 

 of limestone, and is about 1500 feet thick ; the highest part 

 consists entirely of fine shales with thin beds of nodular 

 limestone, and is about 300 feet thick. The last has evidently 

 been formed in much deeper water, for in this small thickness is 

 comprised the whole of the Upper and part of the Middle 

 Devonian. 



Palseontologically, however, as many as seven subdivisions can 

 be distinguished in the following descending order : 



(7. Shales of Rostellec. Black shales with nodular layers, Torno- 

 . I ceras simplex, Bactrites, Entomis, Posidonomya venusta, and 



Buchiola retrostriata. 



&.i 6. Shales of Traouliors. Green and brown, with Rhynehonella 

 cuboides, Rh. pugnus, Pentainerus globus, and Productus sub- 

 aculcalus. 



f5. Slates of Porsguen. Greenish brown, with layers of calcareous 

 ^ I nodules yielding many fossils of Middle Devonian species. 



S 1 4. Grey wacke of Fret. Gritty calcareous slates with Phacops Potieri, 



Spirifer par adoxus, and Sp. auriculatus. 

 '3. Grey wacke of Faou. Grits and slates with Spirifer hystericus and 



C'honetes sarcinulatn, and limestone with Athyris undata. 

 2. Sandstone of Gahard, white, with Orthis Afonnitri and Spirifer 



Pellicoi about 300 feet thick. 



1. Plougastel Beds. Alternating beds of hard dark-green quartz- 

 itic grit, and rough greenish slates. Fossils rare, Bellerophon 

 trilobatus. Orthoceras planiseptatum, Pteria Iccvis, Spirifer 

 ocfoplicatus. 



In this succession the Plougastel Beds appear to correspond 

 with our Dartmouth slates and Glengarriff grita ; the Gahard and 

 Faou Beds correspond to our Meadfoot Beds and Staddon grit-. 

 The Middle Devonian Series is about 800 feet thick, but is very 

 different from that of South Devon, both it and the Upper Devonian 

 having a greater resemblance to the beds in Cornwall. 



In the eastern basins of Laval and Angers another Devonian 

 facies is found. Combining the work of M. .Khlert and the 



