6o CAMBRIAN, 



MIDDLE CAMBRIAN [Scdgzvick). 

 UPPER CAMBRIAN {Hicks). 



This group is divided as follows : — 



2. Tremadoc Slate Series. 

 I. Lingula Flag Series. 



Being marked by the preponderance of Olenus, it has been 

 termed the Olenidian division by Prof. C. Lapworth.^ 



The following subdivisions have locally been made in the 

 group : — 



North Wales. Malvern. Shropshire. 



Tremadoc f rr, , 



Slate ^^f^*" 

 c • j Loivcr Dictyonema Shales. 



Lingula ( Dolgelly Beds Malvern Black Shales Shineton Shales, 



Flag < Ffestiniog Beds Hollybush Sandstone. 



Series. ( Maentwrog Beds. 



The total thickness of the Middle Cambrian rocks has been 

 estimated at about 6000 feet. The Rev. J. C. Ward was of opinion 

 that portions of the Skiddaw Slates might be paralleled with the 

 Lingula Flag Series and Tremadoc rocks (see p. 78). 



LINGULA FLAG SERIES. 



These beds were named by Mr. E. Davis in consequence of his 

 discovery in 1845 of Lingula {Li7igulelta) Davisii in these rocks 

 near Tremadoc. They consist of slaty and shaly beds with grits 

 and hard sandstones, often much altered by the igneous rocks 

 intruded among the strata. Where well developed they attain a 

 thickness of from 5000 to 6000 feet ; but half this estimate is 

 sometimes considered sufficient. They were separated from the 

 Tremadoc rocks in 1847 ^7 Sedgwick, under the name of the 

 Ffestiniog group. 



The fossils are noted in the accounts of the subdivisions of the 

 strata, but it may be mentioned that a ' pod-shrimp' {^Hynie7iocaris) 

 and many Trilobites make their appearance in them. 



Many dykes and intrusive bosses of igneous rock penetrate the 

 beds : some may be seen in the Ffestiniog Slate Quarries. 



The Lingula beds are well developed in Caernarvonshire and 

 Merionethshire, occurring near Bangor and Portmadoc, and rang- 

 ing from the mouth of the Barmouth estuary to the north-east ; 



1 G. Mag. 1 88 1, p. 260. 



