these are new forms excepting the
first, and the last is a new genus. It is a singular fact that the
_Neritina_ is now living in the West India waters, and the species found
at Pebas retains its peculiar markings. So that we have some ground for
the supposition that not many years ago there was a connection between
the Caribbean Sea and the Upper Amazon; in other words, that Guiana has
only very lately ceased to be an island. There is no mountain range on
the water-shed between the Orinoco and the Negro and Japura, but the
three rivers are linked by natural canals.[164] Interstratified with the
clay deposit are seams of a highly bituminous lignite; we traced it from
near the mouth of the Curaray on the Rio Napo to Loreto on the Maranon,
a distance of about four hundred miles. It occurs also at Iquitos. This
is farther testimony against the glacial theory of the formation of the
Amazonian Valley. The paucity of shells in such a vast deposit is not
astonishing. It is as remarkable in the similar accumulation of reddish
argillaceous earth, called "Pampean mud," which overspreads the Rio
Plata region.[165] Some of the Pampa shells, like those at Pebas, are
proper to brackish water, and occur only on the highest banks. The
Pampean formation is believed by Mr. Darwin to be an estuary or delta
deposit. We will mention, in this connection, that silicified wood is
found at the head waters of the Napo; the Indians use it instead of
flint (which does not occur there) in striking a light. Darwin found
silicified trees on the same slope of the Andes as the Uspallata Pass.

[Footnote 162: _A Journey in Brazil_, p. 250, 411, 424. Again, in his
Lecture before the Lowell Institute, 1866: "These deposits could not
have been made by the sea, nor in a large lake, as they contain no
marine nor fresh-water fossils."]

[Footnote 163: These interesting fossils are figured and described in
the _Am. Journal of Conchology_.]

[Footnote 164: "The whole basin between the Orinoco and the Amazon is
composed of granite and gneiss, slightly covered with debris. There is a
total absence of sedimentary rocks. The surface is often bare and
destitute of soil, the undulations being only a few feet above or below
a straight line."--Evan Hopkins, in _Quart. Jour. Geol. Soc_., vol. vi.]

[Footnote 165: See Darwin on the absence of extensive modern
conchiferous deposits in South America, _Geological Observations_, pt.
iii., ch. v.]

The climatology of the