ootnote 160: Messrs. Myers and Forbes found this red clay on the
Negro, most abundantly near Barcellos; also in small quantities on the
Orinoco above Maipures. The officers of the "Morona" assured us that the
same formation was traceable far up the Ucayali and Huallaga. This clay
from the Amazon, as examined microscopically by Prof. H. James Clark,
contains fragments of gasteropod shells and bivalve casts. The red earth
of the Pampas, according to Ehrenberg, contains eight fresh-water to one
salt-water animalcule.]

[Footnote 161: "On the South American coast, where tertiary and
supra-tertiary beds have been extensively elevated, I repeatedly noticed
that the uppermost beds were formed of coarser materials than the lower;
this appears to indicate that, as the sea becomes shallower, the force
of the waves or currents increased."--Darwin's _Observations_, pt. ii.,
131. "Nowhere in the Pampas is there any appearance of much superficial
denudation."--Pt. iii., 100.]

It is a question to what period this great accumulation is to be
assigned. Humboldt called it "Old Red Sandstone;" Martius pronounced it
"New Red;" Agassiz says "Drift"--the glacial deposit brought down from
the Andes and worked over by the melting of the ice which transported
it.[162] The Professor farther declares that "these deposits are
fresh-water deposits; they show no sign of a marine origin; no
sea-shells nor remains of any marine animal have as yet been found
throughout their whole extent; tertiary deposits have never been
observed in any part of the Amazonian basin." This was true up to 1867.
Neither Bates, Wallace, nor Agassiz found any marine fossil on the banks
of the great river. But there is danger in building a theory on negative
evidence. These explorers ascended no farther than Tabatinga. Two
hundred miles west of that fort is the little Peruvian village of Pebas,
at the confluence of the Ambiyacu. We came down the Napo and Maranon,
and stopped at this place. Here we discovered a fossiliferous bed
intercalated between the variegated clays so peculiar to the Amazon. _It
was crowded with marine tertiary shells!_ This was Pebas _vs_.
Cambridge. It was unmistakable proof that the formation was not drift,
but tertiary; not of fresh, but salt water origin. The species, as
determined by W.M. Gabb, Esq., of Philadelphia, are: _Neritina pupa,
Turbonilla minuscula_, _Mesalia Ortoni_, _Tellina Amazonenis_, _Pachydon
obliqua_, and _P. tenua_.[163] All of