142 STRATIGRAPHICAL GEOLOGY 



In the soutli of Brittany the Llandilo Series begins with the 

 sandstone of Chatelier (see Fig. 44), which is overlain near 

 Poligny by the black slates of Kiadan, the latter containing 

 Trinucleus Pongerardi, Acidaspis Buchi, Illcenus Beaiimonti, and 

 Calymene arago. In Northern Brittany the series is largely 

 composed of sandstone. 



The sandstones of Eedon and St. Germain-sur-Ille represent 

 part of the Bala Series, but as they contain few fossils and pass up 

 into Silurian sandstones it is difficult to separate the one from the 

 other, except in the west (Crozon, near Brest), where the two sand- 

 stones are separated by a band of limestone yielding Orthis Actonice 

 and Illcenus Munieri. 



In Normandy the facies is somewhat different and the general 

 succession is as follows : >25 



Feet. 

 Bala Green slates with Trinucleus Grenieri .... 200 



Llandilo Gres de May with Homalonotus, etc. .... 500 



Llanvirn Blue slates with Calymene Tristani .... 300 



A - a /Gres armoricain ........ 400 



' m g ^Gres felspathique, thinning out S. and E. . . variable 



It will be seen that in most parts of this area the total thick- 

 ness is not more than 1500 feet, for the Gres felspathique is only 

 found in the west of the Cotentin and is overlapped to south and 

 west by the Gres armoricain. The overlying Calymene slates 

 correspond with those of Sion and Angers and generally have a 

 bed of ironstone at the base. 



The Gres de May has recently been subdivided into 4 zones 

 characterised by certain species ; these in descending order are : 



4. Sandstones with Conularia pyramidata. 



3. Shales and grits with Trinucleus Bureaui and Cal. Tristani. 



2. Sandstone with Homalonotus (Vicaryi and 3 other sp. ). 



1. Sandstones with Dalmanites armoricanus and Homalonotus. 



Orthis budleighensis is another common fossil in the lower part 

 of the Gres de May. The highest member of the Ordovician, the 

 shale with Trinucleus Grenieri, is not always present ; it is found 

 near Cap la Hague and has a bed of sandstone above it, and thus 

 it is possible that elsewhere it is represented by some of the highest 

 beds of the Gres de May. 



In connection with the Ordovician of Normandy brief mention 

 may be made of the area of Ordovician rocks which has long been 

 known to exist in Cornwall. The area occupied by the older 

 Palaeozoic rocks in the south-west of Cornwall has recently been 

 resurveyed, but only a small portion of it has been proved to be 



