THE ORDOVICIAN SYSTEM 



141 



Cystidean and Asaphus limestones (Llanviniian) of Sweden and 

 Itu.-ia, species common to both being Nileut armadillo and 

 Chasmops cf. Odini ; while others occur in Canada, the genera 

 Ilutlifiurus and Bathyurellus being essentially American. 



The overlying conglomerate has the appearance of lying 

 uin uiiformably on these upper beds, but it is lithologically similar 

 to the Leenarse grits, and is probably of the same age. 



D. SOME EUROPEAN AREAS 

 1. France 



To British geologists the most important region in France 

 where Ordovician rocks are exposed is that of Brittany, Normandy, 

 and Anjou. In this region they lie in the long synclinal troughs 

 which run roughly from west to east and are most fully developed 

 in the central part of the region, from Grandchamp north of 

 Vannes by Redon and Chateaubriand to Angers in Maine-et- 

 Loire. 24 Along this tract the following general succession has been 

 established (see Fig. 44), though some of the subdivisions are not 

 continuous throughout its extent. 



j, I ( Limestone of Rozan with Orthis actonce . \ 



\ Sandstones of Redon and St. Germain (Diplograpti) J 



TI j-i ( The Riadan slates Trinudeus Ponqerardi 



Llandll I Chatelier sandstone ... 



Llanvirn The Sion slates with Placoparia . 



. /The Armorioan sandstone 



' \ Felspathic sandstone and conglomerate 



Feet 

 500 



500 



500 

 200 



The felspathic sandstones (Gres felspathique) are poorly developed 

 in the south of Brittany, but are very thick in the north, from 

 500 to 1000 feet, and have clearly been formed from the erosion 

 of Archaean and granitic rocks during the interval between the 

 Cambrian and Ordovician sedimentation in this part of France. 

 They may indeed be partly of Tremadoc age (see p. 99). 



The Armorican sandstone (Gres armoricain) consists of hard 

 white sandstones with few fossils, chiefly Lamellibranchs of the 

 genera Redonia, Ctenodonta, and Actinodonta and annelid tracks 

 (Scolithes, Bilobites). It also is much thicker along the northern 

 outcrops than along the southern, where it is sometimes less than 

 100 feet Eastward, in Anjou, it is largely represented by shales. 



The Sion slates generally have a bed of oolitic ironstone at 

 their base, and the slates contain graptolites of the geminus and 

 Murchisoni types with the trilobites Placoparia Zippei, Asaphus 

 Guettardi, and Calymene Tristani. 



