374 COMPAEISONS. 



All these members contain fossils, which correspond very well with one another. We 

 will specify a few of one group, as they may be appropriately noticed here. The " argil- 

 laceous schists," C, of Bohemia, or the "Zone Primordiale" of Barrande, are especially 

 characterized by trilobites of the genera Paradoxides, Conocephalus, Arionellus, etc. The 

 same fossils are found in the Lingula flags of Wales, and the best authorities agree in 

 considering the Bohemia group C, and the Lingula flags as equivalent, and as forming 

 the lowest member of the Lower Silurian. The best authorities also agree in regarding 

 the Potsdam sandstone equivalent to the Bohemia group C, and the Lingula flags, on ac- 

 count of its fossils, Conocephalites, Dikelloc&phalus, and Menocephalus. 



There are three views respecting the age of the Georgia slate. 1. Professor Emmons 

 says it is the uppermost member of the Taconic system, and that the Taconic system is 

 stratigraphically below the Potsdam sandstone, that is to say, that the Taconic system is 

 Cambrian. Upon pages 90, 91, of Part V. of American Geology, the Taconic system is di- 

 rectly compared with the Skiddaw slates of Cumberland. In opposition to this view, we 

 would say that the Georgia slate rests conformably upon the Red Sandrock series, as is 

 shown in Fig. 257, and its fossils rank it as Lower Silurian rather than Cambrian. 



2. M. Barrande and Sir W. E. Logan, judging from palaeontological evidence, regard 

 the Georgia slate as equivalent to the Primordial Zone C, of Bohemia, or very nearly the 

 Potsdam sandstone of North America. (See other letters, which follow.) 



3. The stratigraphical view of the Georgia slate, which has been so ably defended by 

 Professor Hall, seems to demand for it a place either above or equivalent to the Oneida 

 conglomerate. This view we now proceed to illustrate by a section through the Hudson 

 River group, Red Sandrock series, Georgia slate, etc., in St. Albans. 



FIG. 257. 



Section through St. Albans. 



Explanation of Fig. 257. 



A Hudson River shales, St. Albans Point, dip 5 E. F Georgia slate (argillo-micaceous), dip 45 E. 



B Dove-colored limestone of the upper part of the Hud- G Brecciated quartz rock, and quartz rock, on Aldis Hill. 



son River Group, dips about 25 E. H Georgia slate (clay slate), dip 45 E. 



C Red sandrock series, average dip 5-10 E. K Quartz rock, dip 30 E. 



D Georgia slate, dip 15-20E. L Talcose schist, and talcose conglomerates. 



E Conglomerate of limestone pebbles, dip 42 E. M Village of St. Alban's Bay. 



S Village of St. Albans. N Stratigraphical position of Barrandia Thompsoni. 



Fig. 257 represents a section of the rocks from St. Albans Point to the east part of the 

 town, as far as the talcose conglomerates. At the west end of the section the rock is 

 without doubt the shales of the Hudson River group of the Lower Silurian. The dove- 

 colored limestones above we have described as a part of the same group. The Red Sand- 

 rock series certainly rests directly upon the Hudson River Group, and the angle of their 

 inclination is very small. D, E, F, and H, in the figure, are all parts of the Georgia slate, 

 and perhaps G is also, though we have described it elsewhere differently, on account of its 

 lithological character. The dip of the brecciated quartz rock on Aldis Hill is probably 



