TACONIC QUESTION IN GEOLOGY. 143 



Hypozoic gneiss system, aud a younger one of crystalline schists, which he called Azoic 

 and placed beneath the horizon of the Scolithus sandstone. The views of H. D. Rogers, 

 in 1858, with regard to the crystalline rocks of the Atlantic belt, were thus, as I have else- 

 where said, "a return to those held by Eaton and by Emmons, but were in direct opposi- 

 tion to that of Mather, which had been adopted by Logan and the present writer," -' and, 

 so far as regards the "White Mountains, were maintained by the Messrs. Rogers themselves 

 in 1846. 



§ 181. Henry D. Rogers died in 186Y, but his venerable brother, William B. Rogers, 

 survived till 1882, aud fully shared the views set forth by the former in 1858, as to the 

 pre-paleozoic age of the great groups of crystalline rocks. His careful and extended 

 studies in Virginia during many years had convinced him of the fallacy of the metamor- 

 phic hypothesis of Mather. In a sketch of the geology of that state, contributed by him 

 as late as 18*78 to Macfarlane's " Geological Railroad Guide," Rogers makes it plain that 

 the crystalline rocks of that region are all pre-paleozoic, and older than what he calls the 

 Primal or Potsdam group. This he describes as lying on the western slope, and in the 

 west-flanking hills of the Blue Ridge, " often by iuA^rsion dipping to the south-east, in 

 seeming conformity, beneath the older rocks of the Blue Ridge, but often, also, resting 

 unconformably upon or against them." These older rocks, he tells us, "comprise masses 

 referable probably to Hnronian and Laurentian age," and, farther, he informs us that the 

 letters, A, B, C and D, nsed in his tabular view, " mark four rather distinct groups of 

 Archean rocks found in Virginia, of which the first three may probably be referred to the 

 Laurentian, Hnronian and Montalban periods respectively, and the fourth to an inter- 

 mediate stage, — the Noriau or Upper Laurentian." 



§ 182. It should here be remarked that this Primal group of the valley of Virginia, 

 also called by Rogers, Lower Cambrian, is no other than the base of the Lower Taconic 

 series, which he continued to regard as in some sense the representative of the Cambrian 

 Potsdam of the Adirondack region. In this connection, as showing the relations of this 

 group to the crystalline rocks, and the apparent inverted succession, I venture to make the 

 following extracts from a letter from W. B. Rogers, written to me in 18*7*7, for publication 

 in my volume on Azoic Rocks, after an examination with him of some forty unpublished 

 transverse sections, made across the Blue Ridge during his geological survey of Virginia. 

 In many of these sections " illustrating the position of the Lower Cambrian, (our Primal 

 conglomerate, etc.,) in their contact with the crystalline and metamorphic rocks of the 

 Blue Ridge in Virginia," " the unconformity of the Cambrian upon and against these crys- 

 talline and metamorphic rocks is unmistakable and conspicuous ; the lower members of 

 the Primal being seen to rest upon the slope of the Ridge, with north-west undulating- 

 dips, on the edges of the steeply southeastward-dipping older rocks. In other cases, the 

 Primal beds, thrown into south-east dips in the hills which flank the Blue Ridge, are made 

 to underlie, with more or less approximation to conformity, the older rocks forming the 

 central mass of the mountain." Here follow details as to localities, for which the reader 

 is referred to the letter as published.'-" 



§ 183. While, therefore, the brothers Rogers and others with them held, and still hold, 



" Hunt, tlie Hi.storj' of Pre-Cambrian Rock.s in America and Europe, 1880, Amer. Jour. Science, xix, p. 272, 

 ''" Hunt, Azoic Rocks, p. 198. 



