Rocks. 59 



rocks will be seen by referring to fig. 62, p. 322. They 

 form there the lowest central strata of a broad anti- 

 clinal curve. They are also well seen in the Passes of 

 Llanberis and Nant Ffrancon in Carnarvonshire, where 

 the celebrated slate quarries of Penrhyn and Llanberis 

 lie in these strata. The slates are purple, purplish- 

 blue, and green; and associated with them are beds 

 of greenish and grey grits and conglomerates. It is 

 important to observe that at Llanberis the latter contain 

 numerous water- worn pebbles of felspathic traps, jasper, 

 greenstone, black and purple slate, &c., so that these, 

 forming part of the oldest rocks of Wales, have been 

 partly derived from pre-existing rocky lands, simi- 

 lar to those that now form the neighbouring Silurian 

 country, but no visible trace remains of this more ancient 

 physical geography, except the pebbles in the con- 

 glomerate. In Anglesea the equivalent rocks are meta- 

 morphic chlorite and mica-schist and gneiss. 



Cambrian strata also occur in the hills of the 

 Longmynd of Shropshire, where the strata stand nearly 

 on end. They consist of green, grey, and purple slaty 

 rocks, grits, and conglomerates. The only traces of 

 fossils yet discovered in these consist of worm-burrows, 

 and a trilobite, Palceopyge Ramsayi. 



FIG. 13. 



Section across the Longmynd and Shelve country. 

 Shelve Longmynd 



1. Cambrian grits and slates. 2. Lingula flags of the Stiper stones. 

 3. Tremadoc beds. 4. Llandeilo and Caradoc rocks with igneous 

 interstratifi cations. 5. Upper Llandovery and Wenlock rocks. 



At St. David's, in North Pembrokeshire, in equiva- 

 lent strata, Mr. Hicks found the following fossils in 



