Crosby.] 488 [April 7, 
Mr. Derby calls the felsitic group Huronian, evidently believing 
that the felsites are original; and he says that on the Trombetas 
River it is overlaid unconformably by sandstones containing Upper 
Silurian fossils. Lower Silurian (Cambrian) rocks have never been 
certainly identified in Brazil; but the Tocantins or itacolumite series 
was assigned to this chronological position by Prof. Hartt; and Mr. 
Derby, having shown by elimination that it is below the Upper Silu- 
rian and above the Huronian, concurs in the same conclusion. The 
semi-crystalline character of this itacolumite or hydromica schist 
series, the complete absence in it of organic remains, and the fact 
that its stratigraphical no less than its lithological features con- 
nect it much more closely with the underlying felsites and gneisses 
than with the Upper Silurian or any part of the Paleozoic system, so 
far as known, render it extremely improbable that this conclusion 
will be sustained by future investigation. And, furthermore, in my 
opinion, Mr. Derby’s reasoning is defective, inasmuch as the exist- 
tence of extensive systems of stratified rocks between the Huro- 
nian and the base of the Cambrian is now generally recognized; 
these intervening terranes being, of course, those bearing the names 
Montalban and Taconian. 
The lithological and stratigraphical resemblance between the 
itacolumite, schist and limestone series of Brazil and the Taconian 
system of Eastern North America, including as typical examples of 
the latter the quartzite, saccharoidal limestone and hydromiea schist 
of Berkshire County, Massachusetts, and the flexible sandstones and 
associated schists and limestones of the Carolinas, is so striking that 
it must be generally admitted; and I need not enter here into any 
detailed comparisons to enforce this point; the more especially since 
I have been anticipated in this correlation by so high an authority as 
Dr. Hunt. Neither their composition, structure, nor, so far as known, 
their relations to the subjacent and superjacent formations afford any 
reasonable ground for questioning the chronological equivalence of 
these two great series of rocks; and in both continents geologists are 
generally agreed that they lie along the border line between the Eozoic 
and Paleozoic, opinion differing mainly on the question as to whether 
they form the top of the former or the base of the latter. Although, 
as the preceding shows, I am not unmindful that the pre-Cambrian | 
age of the Taconian system is disputed by many geologists, yet 
there are several points in the geology of South America having an 
important bearing on this question to which I desire to direct at- 
tention. 
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