86 STKATIGRAPHICAL GEOLOGY 



Harlecli dome (see Fig. 19). They are well exposed near Dolgelly 

 and Ffestiniog and were studied in 1867 by Mr. Belt, who sub- 

 divided them into three groups, each characterised by a certain 

 assemblage of Trilobites and other fossils. More recently they 

 have been examined by Professor Lapworth and Dr. Wilson (op. 

 cit.), who appear to accept and confirm Belt's main divisions, which 

 are as follows with thicknesses corrected by the later estimates. 



Feet. 



Dolgelly / Soft black shales with Peltura scarabccoides . . 300 



Beds \Hard blue slates with Parabolina spinulosa . . 300 



Ffestiniog f Bluish-grey flags with Olenus micrurus ... 50 



Beds I Grey micaceous flags with Hymenocaris . . . 3000 



Maentwrog f Black slates and flags with Olenus gibbosus . . 1600 



Beds ^Dark-grey flags and shales with 0. catarades . . 1000 



About 6250 



The lower portion of the Maentwrog Beds has been termed 

 the Vigra Beds by Messrs. Lapworth and Wilson, and the higher 



14 13 12 11 {/ 10 8765 4 3 2 la 1 



Pio. 20. SECTION ACROSS THE PENRHYN PROMONTORY (after Salter). 



12, 13. Llanyirniari. 2 to 4. Lower Tremadoc. 



11. Arenig Grit. 1 and la. Lingula fla-^s. 



5 to 10. Upper Treiradoc. g. Intrusive dyke of igneous rock. 



There is probably a fault below the letter F. 



portion the Pen Rhos Beds. The former contain beds of fine 

 siliceous grit from 2 to 3 feet thick, while in the latter such grits 

 are only from to 2 inches thick. Two species of Olenus and 

 Agnostus pisiformis are the characteristic fossils. 



The Ffestiniog Group is a thick mass of flagstones with only a 

 few bands of black shale ; they are usually hard and consequently 

 form higher ground than the underlying shales. The most abund- 

 ant fossil throughout is Lingulella Davisii. 



The Dolgelly Group consists of soft blue and black shales, which 

 appear to have been formed in much deeper water and though 

 only 600 feet probably represent as long a period of time as the 

 whole of the Ffestiniog Beds. They resemble the Alum shales of 

 Sweden. 



The best section of the Tremadoc slates in North Wales is in 

 the cliffs of the Penrhyn promontory south-east of Tremadoc in 

 Merioneth. According to Mr. Salter 8 the succession which can 

 here be seen is as follows : 



