TIIK CAMr.l;: ,.M 81 



series, which, however, is not exposed. Abovi- tln-m rn- blue and 

 purple slates and grits (the Cefu Slate Group) of which ih.-rv inay be 

 1000 feet. These are succeeded by the Rhinoy gritt, which hare a 

 thickness of about 2500 feet These two groups may be regarded 

 as representing the Lower Cambrian. 



Passing under the great mountain mass of the Snowdou range, 

 the Harlech Series again emerges and occupies a considerable area 

 in Carnarvonshire from the coast near Clynnog to Penrhyu and 

 Llanllechid on the north-west, lying on both sides of the axial ridge 

 of Archaean rocks. The basal beds are grita and conglomerates, 

 which contain pebbles of granite, quartz-felsite, and porcellanite 

 derived from the pre-Cambrian rocks described in the preceding 

 chapter. These are succeeded by a thick series of purple and green 

 slates, some of which are worked at the celebrated slate quarries of 

 Penrhyn and Llanberis ; above these again are coarse grits and sand- 

 stones. The total thickness on the eastern side of the anticline 

 near Llanberis is estimated at about 3000 feet. 



The beds exposed in and near the Penrhyn slate quarries were 

 described by Professor T. M'K. Hughes in 1889, from whose paper 

 the following condensed account of the succession is taken : 



Lingula Hags with /.. Davitii. 1 ' 



.S 16. Coarse grits aiid sandstones of Bronllwyd . . . . T 600 



^3 -! 5. Purple aiid bluish-green slates, yielding Conocoryphe viola 



'~*\ in the upper part 1500 



4. Band of fine red grit (15 feetj. 



3. Purple and veined slates (quarried) . 



2. Green sandy slates ......- 



.l. Grits, slates, and basal conglomerate .... 



3140 



Of these beds 1 to 4 may represent the Caerfai Series, but 

 as no fossils have yet been found in them correlation i* un- 

 certain. 



Paradoxidian or Middle Cambrian. This aeria 

 distinguished by the presence of several species of th- TnM.it,- 

 genus Paradox-ides. In Pembrokeshire it was divided ! 

 Hicks into two stages, the Solva Group and the Mentvian Group, 

 Menevia being the ancient name of St. David's. He alao recognised 

 the following subdivisions or zones : 



