Campbell — Rogers's Geology of the Virginias. 369 



into the rocks composing the mountains bounding it on the 

 west." (pp. 209, 210). 



Prof. Rogers nowhere in his State Reports undertakes to 

 subdivide these limestones by the several horizons which he 

 gives them in his notes for Macfarlane's Guide (see pp. 717, 720). 

 Here he recognizes the three subdivisions, 3a, 3b and 3c, 

 which are tolerably well defined in the Great Valley. 



We are aware that Prof. Dana in his " Table of Formations," 

 as revised for Macfarlane's Geological Railway Guide, has re- 

 duced the sub-divisions of the Canadian group to two, Calcifer- 

 ous and Chazy, for which we have no doubt he can present 

 very cogent reasons. In the Virginias this whole group is more 

 remarkable for the paucity than for the abundance of its fossils ; 

 but still enough have been found to mark its place in geological 

 history. Under the guidance, however, of organic remains 

 alone, we have not been able to trace very definite horizons for 

 separate epochs in this Canadian period, but when we call in 

 the aid of lithological features as an additional basis of divis- 

 ion, we find three tolerably well defined formations, blending 

 into one another to some extent it is true, but not more so than 

 is very common in contiguous and conformable limestone beds. 



It is a fact worthy of note that what we call three formations 

 of this period carry with them, throughout their whole extent 

 of 340 miles, such well defined lithological peculiarities as to 

 indicate clearly that they were severally deposited under quite 

 different, though not suddenly changed, circumstances. 



The lowest beds, which we regard as corresponding with the 

 Calciferous (3a) of New York — though containing here far 

 less of calciferous sandstone than of calciferous shale — is well 

 characterized, as exposed in numerous ravines along the west- 

 ern base of the Blue Ridge, by heavy beds of purplish shales, 

 with interstratified beds of dolomitic and siliceous limestones, 

 like that employed at Balcony Falls factory for making 

 hydraulic cement. The shales are both calcareous and ferru- 

 ginous, but become more calcareous and less ferruginous as they 

 are found to rise higher in the series. 



In our notes on the Virginia railroads, prepared for a new 

 edition of Macfarlane's Geological Railway Guide, we have re- 

 tained the Quebec (Levis) (35) as a distinct division because of 

 the comparatively abrupt transition from predominating ferru- 

 ginous shales on the one hand, to predominating ferruginous 

 and .dolomitic limestones on the other, on the surfaces of which 

 we find in some localities numerous fucoidal impressions quite 

 different from any found in the lower series. Here the lime- 

 stone beds increase in thickness, in the regularity of their bed- 

 ding, in their purity as regards silica and alumina, but they 

 carry iron enough to produce by their disintegration dark, fer- 



