164 GEIGER AND KEITH — STRUCTURE OF THE BLUE RIDGE. 



shales and Oswego and Medina sandstones. Along the western side of the 

 Green mountains and southward through New^ York and New Jersey we 

 have Cambrian sandstone and Lower Silurian limestone and shale in succes- 

 sive order, while in the Blue ridge section, described by Messrs. Geiger and 

 Keith, the succession is, limestone, shale, sandstone, as in the Silurian section 

 of New York just mentioned. In the latter case erosion has evidently not 

 cut through the Silurian limestone to the Cambrian ; and the section is that 

 of an overlapping deposit upon a sloping pre-Paleozoic shore-line, similar to 

 that about the Adirondack region. From these facts I think it probable 

 that the interpretation of Messrs. Geiger and Keith is the correct one. 



Professor C. H. Hitchcock : If the authors allow that the reference of 

 the quartzites next the crystallines to the middle Silurian applies only to the 

 region of Harper's Ferry, they may be correct. I understood them, however, 

 to claim the reference of the whole of W. B. Rogers' number 1 to this hori- 

 zon, insisting that no reliance should be placed upon the sections at Balcony 

 Falls and near Christiansburg, where the sandstones or quartzites underlie 

 the lower Silurian limestones. I am familiar with this part of the great 

 valley of Virginia, and should interpret the structure as Rogers and Campbell 

 have done, both by reason of the stratigraphy, and because fragments of the 

 crystalline rocks further east are constituents of the basal conglomerates, 

 which in their turn underlie the limestones. The presence of fragments of the 

 older rocks in the derived sediments affords a better criterion for the deter- 

 mination of the succession of the terranes on the western flank of the Blue 

 ridge than their dips. One can explain the presence of eastern dips by in- 

 versions or faults if necessary, but cannot understand how a composite sedi- 

 ment can be older than its constituent rounded pebbles. Thirty years since 

 our best geologists overlooked this obvious principle in explaining the struct- 

 ure of these same rocks in western New England and referred the quartzites 

 to the Medina ; to-day there is not a single geologist familiar with the ground 

 who would accept the early views of Logan, Hall and Dana in reference to 

 this point. Hence these Harper's Ferry outcrops must represent only local 

 dispositions. 



Major Jed Hotchkiss: Can the authors of the communication inform 

 us concerning the age and relations of the limestones frequently found east 

 of the Blue ridge? 



Mr. Keith : Limestones sometimes occur as small lenses in slate over the 

 Archean area east of the Silurian limestones of the Shenandoah valley. In 

 one case (near Sharpsburg) the Silurian limestones rest on shales which may 

 be Cambrian. 



