LOWER SILURIAN. 



179 



2. European. 



The Primordial or Cambrian rocks of Great Britain outcrop in 

 North and South Wales, and in Shropshire (or Salop), just east of 

 Wales. The lowest rocks of the series are the shales and sandstones 

 of the Longmynd, in Shropshire, and of northern Wales, the maxi- 

 mum thickness of which has been estimated at 28,000 feet. The 

 Penrhyn and Llanberis slates are in the upper part of the series in 

 north Wales, near the Menai Straits. In southwest Wales, there are 

 (1) the Harlech grits, overlaid by (2) the Menevian group. Similar 

 rocks occur in County Wicklow and County Dublin, in Ireland, 

 which are supposed to be of the same age. The Longmynd rocks 

 are the Lower Cambrian of Sedgwick. In northwest Scotland, beds 

 referred to the Cambrian, consisting of red and purple sandstones and 

 conglomerates, overlie unconformably the Archaean. 



The Cambrian rocks of the Longmynd and north Wales are over- 

 laid conformably by the Lingula flags, a series of beds of shale, grit, 

 and sandstone, 3,000 to 4,000 feet thick. The three British divisions 

 of the Primordial are, 1, Lower Cambrian ; 2, Menevian, or Upper 

 Cambrian, corresponding to the American Acadian group, and con- 

 taining species of Paradoxides ; 3, the Lingula flags, or upper part 

 of them, affording, like the American rocks of the Potsdam period, 

 no Paradoxides. 



In Lapland, Norway, and Sweden, there is a Primordial sandstone overlaid by 

 schists, the lowest beds passing at times into a conglomerate ; the regions A, B of the 

 geologist Angelin. In Bohemia, the lowest Primordial beds are schists 1,200 feet 

 thick, called by Barrande Protozoic schists, or the Primordial Zone, and numbered C 

 in his series, — his A, B consisting of schists and conglomerates conformable to C. 

 Until recently, B was thought to contain no trace of life, and therefore to be below the 

 Primordial ; but worm-burrows have been reported to occur in some of these inferior 

 beds. South of Hof, in Bavaria, there are other rocks of the Primordial zone. 



1. Life — 1. Cambrian. — The Longmynd rocks have afforded worm-burrows, the 

 species named Arenicolites didyma. From the Harlech beds of the Upper Cambrian, 



Fig. 276, Oldhauiia antiqua ; 277, 0. radiata ; 278, Lingulella Davisii : 279, Agnostus Rex; 280, 

 Olenus micrurus ; 281, Sao hirsuta {X>a)| 282, Hymenocaris vermicauda (x^). 



many species have been described, including Pteropods, Brachiopods, Phyllopods, 

 Trilobites and Annelid tracks. And from the Menevian, a larger number, among 



