382 CAMBRIAN AND SILURIAN IN EUROPE. [XV. 



The distribution of the Lower and Middle Cambrian rocks 

 in Great Britain may now be noticed. The former, or Bangor 

 group, to which Murchison and the geological survey rot rid 

 the name of Cambrian, and which they sometimes call the 

 Longmynd, bottom or basement rocks, occupy two adjacent 

 areas in Caernarvon and Merionethshire ; the one near Baugor, 

 including Llanberris, to the northeast, and the other, including 

 Harlech and Barmouth, to the southeast, of Snowdon ; this 

 mountain lying in a synclinal between them, and rising 3,571 

 feet above the sea. The great mass of grits or sandstones ap- 

 pears to be at the summit of the group, but in the lower part 

 the blue roofing-slates of Llanberris are interstratified in a series 

 of green and purple slates, grits, and conglomerates. (Some 

 of the Welsh roofing-slates are, however, supposed to belong 

 to the Llandeilo. Mem. Geol. Survey, III. Part II. pages 54, 

 258.) The Harlech rocks in this northwestern region are con- 

 formably overlaid by the 'Menevian, followed by the true 

 Lingula flags, or Olenus beds, of the Middle Cambrian. Upon 

 these repose the Tremadoc slates. 



The third area of Lower Cambrian rocks known in Great 

 Britain is that already described at St. David's in Pembroke- 

 shire, about one hundred miles to the southwest ; and the 

 fourth, that of the Longmynd hills, about sixty miles to the 

 southeast of Snowdon. The rocks of the Longmynd, like 

 those of the other Lower Cambrian areas mentioned, consist 

 principally of green and purple sandstones with conglomerates, 

 shales, and some clay-slates. They occasionally hold flakes of 

 anthracite, and small portions of mineral pitch exude from 

 them in some localities. The only evidence of animal life yet 

 found in the rocks of the Longmynd are furnished by worm- 

 burrows, the obscure remains of a crustacean (Palceopyye L '> un- 

 say), and a form like Histioderma. This latter organic relic, 

 with worm-burrows, and the fossils named Oldhamia, is found 

 on the coast of Ireland opposite Caernarvonshire, in the rocks 

 of Bray Head ; which resemble lithologically the Harlech 

 beds, and are regarded as their equivalents. 



Still another area of the older rocks is that of the Malvrrn 



