172 STKATIGRAPHICAL GEOLOGY 



reason to think that the upper beds of zone 5 are .sometimes 

 concealed by an overlap of zone 6 (Cyrtograptus Lundgreni), which 

 represents the Wenlock limestone. 



The shales overlying this zone are similarly divisible, and Miss 

 Wood has distinguished five zones in the following descending 

 order : 



Feet, 

 a. Zone of Monograptus Lcinticardincnsis, including the 



Aymestry limestone ..... 400 to 500 

 4. ,, Monograptus tumesccns ... . 100 to 220 



3. ,, Monoijraptus scanicus ..... 250 to 100 

 2. , , Monograptus Nilssoni . . . . . ) 

 1. ,, Monograptus milgaris . . . . . j 



300 to 400 



Thus the total thickness of the Salopian Series in the Builth 

 district is about 2600 feet. 



When these beds are followed northward they become more 

 arenaceous, thick felspathic grits and micaceous flags appearing 

 in the Wenlock shales, while the higher beds are gradually 

 replaced by a great mass of sandstones, grits, and flagstones in 

 which fossils are much less abundant. This is the aspect presented 

 by the Salopian Series in Denbighshire, where parts of them have 

 been studied by Mr. Philip Lake, 9 Professor T. M'K. Hughes, 10 Dr. 

 H. Hicks, 11 and by the Misses Elles and Wood 12 (see Figs. 57 and 

 58). Combining the observations and correlations of these authors, 

 we find that the succession may be generalised as follows : 



6. Gritty sandstones and wavy banded concretionary mudstones 



of Moel Ganol. 



'5. Grey banded mudstones and sandstones with Acidasjns HugJiesi,. 

 4. Nantglyn flags with Actinucrinus pulcher, Camarotcechia nucula, 



Dayia, navicula, Ortkoceras Sedgwicki, 0. primozvuin, Cardiola 



interrupta, and Monograptus Nilssoni. 



3. Moel Ferna slates with Monotjraptus priodon and M. Flemingi. 

 2. Pen-y-glog grit with Mcristclla tumida, Strophomena deprcssa, 



Phacops sj)., and Encrinites. 

 1. Pen-y-glog slates with Cyrtograptus Murchisoni, Gladiogra/ifus 



yeinitzianus, Monograptus priodon, and M. vomerinus. 



The slates at the base are the equivalents of the lower part of 

 the Wenlock shales, and the same fossils have been found in beds 

 overlying the Tarannon shales near Conway. The Moel Ferna 

 Beds must include the representative of the Wenlock limestone, 

 but they yield very few fossils. The Nantglyn flags represent the 

 Lower Ludlow Beds, and Miss Elles has recognised two zones in 

 them, a lower zone of Monograptus vulgaris and a higher one of M. 

 Nilssoni. In the valley of the Dee Mr. Lake found some micaceous 

 flagstones which contain Monograptus Leintwardinensis in abundance, 



