1885.] Physical Geography of the Amazons Valley. 29 
6. The alluvial flood-plains of the Upper Amazons are far 
more extensive than those of the lower river, probably attaining 
in some places a width of at least one hundred miles. They are 
covered everywhere with heavy forest which, during a large por- 
tion or the whole of the year are flooded, so that canoes can pass 
freely underneath the branches. On the Lower Amazons the 
alluvial belt varies in width from fifteen to forty miles, and it 
is occupied, in great part, by open meadows which are only 
flooded during the rainy season. 
7. The fauna and flora of the Upper Amazons are exceedingly 
rich in genera and species, and they are almost entirely com- 
posed of forms which are fitted only for a forest life. On the 
Lower Amazons such forms are mingled with others which be- 
long to the open lands, or which are not essentially sylvan; in 
general the species are less numerous than on the Upper Ama- 
zons, and many of them are distinct, but allied or “ representa- 
tive” forms. Those species which are common to the two 
regions are frequently larger and of more rapid growth on the 
Upper Amazons. 
As may be supposed the two regions fade into each other, but 
something like a definite boundary between them is formed by 
the Rio Negro on the north and the Madeira on the south side 
of the Amazons. These, the former with its broad expanse of 
water, the latter with its immense flood-plain, are almost impassa- 
ble barriers to the migration of species. They, together with the 
main river, divide the whole Amazons valley into four parts, each 
of which is characterized by a pretty large number of animals 
and plants. The other great tributaries may limit lesser groups 
of species, and the great flood-plain has a perfectly distinct as- 
semblage of animals and plants which, in their turn, differ essen- 
tially on the Upper and Lower Amazons. 
Let us now, for the moment, leave the Solimoens and confine 
our attention to the region east of the Rio Negro and Madeira. 
The valley of the Lower Amazons is limited on the north by the 
mountain range which separates British and Dutch Guiana from 
Brazil. Most of these mountains are table-topped; and they are 
clearly remains of a great elevated plain. The region, two or 
three hundred miles wide, which separates them from the Ama- 
zons, is very imperfectly known, but it appears to be almost en- 
tirely occupied by a less elevated plain, edges or spurs of which 
