CHAPTER VI 



THE ORDOVICIAN SYSTEM 



A. NOMENCLATURE AND DIVISIONS 



THE grounds on which this system has been established have 

 already been explained (see p. 70). It is the Upper Cambrian of 

 Sedgwick and the Lower Silurian of Murchison and the Geological 

 Survey. The system has usually been divided into three series 

 with the following names : 



3. Upper Ordovician or Bala Series. 



2. Middle Ordovician or Llandilo Series. 



1. Lower Ordovician or Arenig Series. 



The names are taken from places in Wales where the several 

 series are well developed, i.e. the Arenig Mountains in Merioneth, 

 the town of Llandilo in Carmarthenshire, and the town of Bala 

 in Merioneth. This succession was first established in North 

 Wales by Sedgwick. Part of it was studied at the same time in 

 Shropshire and Carmarthen by Murchison, who described the local 

 equivalents of the Bala Series under the name of Caradoc 

 Sandstone, and after their identity had been demonstrated the 

 beds were for a time called the Bala and Caradoc Seriea 



These divisions, however, were not separately mapped either by 

 Murchison or by the Geological Survey, so that they were never 

 clearly marked off from one another, nor was any definite horizon 

 recognised as the base of the Llandilo Seriea or as that of the 

 Bala Series. The first to establish divisions on a strictly palteonto- 

 logical basis was Dr. H. Hicks 1 who in 1875 recognised both the 

 Arenig and Llandilo Beds in Pembrokeshire, and subsequently 

 found that they were separated by a set of black shales whiflfc 

 contained a new and distinct fauna. 2 These he called the Llanvirn 

 Beds, but for a long time this group was regarded as a superfluous 

 division, only embodying what should or could be classified as the 

 Upper Arenig and the Lower Llandilo stages. 



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