GEOGNOSY OF THE APPALACHIANS. 475 
the genus Paradoxides, which also oceupies a lower division in the 
primordial paleozoic rocks of Bohemia (Barrande’s stage C), 
the greater part of which are regarded as the equivalent of the 
Olenus zone of Sweden and the Potsdam of North America. The 
Lingula flags of Wales belong to the same horizon, and it is at 
their base, in strata once referred to the Lower Lingula flags, that 
the Paradoxides is met with. These strata, for which Hicks and 
Salter, in 1865, proposed the name of the Menevian group, are 
regarded as corresponding to the lower division of the Alum slates, 
and, like it, contain a fauna not yet recognized in the basal rocks 
of the New York system. We here approach the debatable land 
between the Cambrian and the Silurian of the British geologists. 
The Cambrian, as originally claimed by Sedgwick, included in its 
upper division the Middle and Upper Lingula flags, with the over- 
lying Tremadoc slates, to the base of the Llandeilo rocks, and may 
be regarded as equivalent to the Potsdam, Calciferous and Levis 
formations ; while in the Lower Cambrian were embraced the Lower 
Lingula flags and the Upper and Lower Longmynd rocks, corres- 
ponding respectively to the Harlech grits and the Llanberis slates. 
A portion of the Cambrian has, however, been claimed for the 
Silurian by Murchison, who draws the dividing line at the top of 
the Longmynd rocks, leaving the three divisions of the Lingula 
flags in the Silurian. Lyell, on the contrary, remarks that the 
Menevian beds, which were, on lithological grounds, made by 
Sedgwick a part of the Lower Lingula flags, have been shown 
by Hicks and Salter to be very distinct from these paleontologi- 
cally; and, while he includes the Menevian in the Lower Cam- 
brian, refers the whole of the Lingula flags to the Upper Cambrian. 
Lyell therefore admits the whole of the Cambrian system as 
originally defined by Sedgwick, and the same classification is now 
adopted by Linarsson, in Sweden, where in Westrogothia, the Cam- 
brian rocks, (resting unconformably on the crystalline schists to be 
noticed farther on), are overlaid conformably by the orthoceratite 
limestones, which are by him regarded as forming the base of the 
Silurian, and as the equivalent of the Llandeilo rocks of Wales. 
The total thickness of these lower rocks in Sweden, including the 
representatives of the Lingula flags, the Menevian beds and an 
underlying fucoidal (Eophyton) sandstone, is only three hundred 
feet, while the first two divisions in Wales have a thickness of 
five to six thousand, and the Harlech grits and Llanberis slates 
