424 A JOURNEY IN BRAZIL. 
allow the accumulation of these materials, and its sub- 
sequent elevation. I reject this explanation for the s'imple 
reason that the deposits show no sign whatever of a 
marine origin. No sea-shells, nor remains of any marine 
animal, have as yet been found throughout their whole 
extent, over a region several thousand miles in length 
and from five to seven hundred miles in width. It is 
contrary to all our knowledge of geological deposits to 
suppose that an ocean basin of this size, which must have 
been submerged during an immensely long period in order 
to accumulate formations of such a thickness, should not 
contain numerous remains of the animals formerly inhab- 
iting it.* The only fossil remains of any kind truly belong- 
ing to it, which I have found in the formation, are leaves 
taken from the lower clays on the banks of the Solimoens 
at Tonantins ; and these show a vegetation similar in 
general character to that which prevails there to-day. 
Evidently, then, this basin was a fresh-water basin ; these 
deposits are fresh-water deposits. But as the valley of 
* I am aware that Bates mentions having heard that at Obydos cal- 
careous layers, thickly studded with marine shells, had been found inters trat- 
ified with the clay, but he did not himself examine the strata. The Obydos 
shells are not marine, but are fresh-water Unios, greatly resembling Aviculas, 
Solens, and Areas. Such would-be marine fossils have been brought to me 
from the shore opposite to Obydos, near Santarem, and I have readily rec- 
ognized them for what they truly are, fresh-water shells of the family of 
Naiades. I have myself collected specimens of these shells in the clay-beds 
along the banks of the Solimoens, near Teffe, and might have mistaken 
them for fossils of that formation had I not known how Naiades burrow in 
the mud. Their resemblance to the marine genera mentioned above is very 
remarkable, and the mistake as to their true zoological character is as nat- 
ural as that by which earlier ichthyologists, and even travellers of very recent 
date, have confounded some fresh-water fishes from the Upper Amazons, of 
the genus Pterophyllum (Heckel), with the marine genus Platax. 
