260 DR. THOMAS STERRY HUNT ON THE 
divisions, the lower of which, as he showed, was distinct from the upper ; as appeared by 
its different geographical distribution. That these two divisions of the Hudson-River 
group were supposed by him to be associated with a still older series, lithologically resem- 
bling them, would appear from Vanuxem’s language, when he wrote of “the difficulty of 
Separating or distinguishing the slaty and schistose members of the Hudson-River group 
from those of greater age, with which, along their eastern border, the two (sic) are more or 
less, really or apparently blended.” 
§ 114. Hall,while thus admitting the existence of an apparent nnconformability between 
the older and the newer fossiliferous rocks in this disturbed region, fell back on Logan’s 
explanation, and imagined the juxtaposition of the two series to be effected by a break of 
the strata, with an uplift on the eastern side; by which the rocks of pre-Trenton age 
were brought up, and were sometimes found in contact with the Trenton or Utica divisions, 
at others with the Loraine, and, perhaps, even with the still higher beds of the Oneida. I 
have elsewhere discusssed at length this hypothesis of a single great fault, with an upthrow 
of 7,000 feet, imagined by Logan to extend from Alabama to the north-east extremity of 
the continent, in Gaspé ; and having shown its great improbability both geographically and 
stratigraphically, have maintained, for ten years past, the simpler explanation of an uncon- 
formity between the First Graywacke and the succeeding numbers of the paleozoic series. 
(Azoic Rocks, pp. 121-125). Evidences are there given that movements of the earth’s crust 
in these regions immediately preceded the Trenton age, and that upon the folded, eroded 
and submerged strata of the First Graywacke, as upon the Taconian and still older series, 
there were subsequently deposited the Trenton limestones. Where these limestones were 
afterwards removed by denudation, or where, to the eastward, they thin out and disappear, 
we may expect to find in direct superposition upon these older rocks, the Loraine or the 
succeeding Oneida strata. 
§ 115. In 1863. Logan having followed southward into Vermont, the Gray wacke-belt, 
to which he had then given the name of the Quebec group, proceeded in company with 
James Hall, to examine the same rocks in eastern New York; where they were then des- 
cribed by him, as they had been by Emmons, as sandstones and conglomerates, generally 
with argillites, sometimes red and green, and with limestones, often schistose or concretion- 
ary, including the Sparry Lime-rock of Eaton. All of these were now declared by Logan 
to belong to the Quebee group, which was said by him to occupy nearly the whole of 
Columbia, Rensselaer and Dutchess counties ; the Sillery division being largely displayed 
in the first-named of these, but scarcely appearing south of it. To the westward, in 
approaching the river Hudson, these rocks were declared to be replaced by lithologically 
distinct and more recent strata, referred to the Loraine shalés; a narrow belt of which was 
traced along the east side of the river to a point alittle above Hyde Park, where the boun- 
dary of the two divisions crosses to the west bank. The strata on both shores, from 
thence down to the gneiss of the Highlands, were referred by Logan to the Quebec group. 
§ 116. Logan, however, as we have seen, assumed the Sillery sandstone to be the summit 
instead of the base of the First Graywacke ; and when he became, at this time, acquainted 
with the underlying Taconian marbles in Vermont, and farther southward, imagined them 
to be his Levis and Lauzon divisions in an altered condition, and thus described them as 
members of the Quebec group. It yet remains to determine in this region the limits 
between the Taconic and the First Graywacke. We now know, moreover, from the disco- 
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