of low hills, interrupted here and there, but extending in evident
connection from Almeirim through the region of Monte Alegre to the
heights of Obidos. These hills have attracted the attention of
travellers, not only from their height, which appears greater than it
is, because they rise abruptly from an extensive plain, but also on
account of their curious form, many of them being perfectly level on
top, like smooth tables, and very abruptly divided from each other by
low, intervening spaces.[D] Nothing has hitherto been known of the
geological structure of these hills, but they have been usually
represented as the southernmost spurs of the table-land of Guiana. On
ascending the river, I felt the greatest curiosity to examine them; but
at the time I was deeply engrossed in studying the distribution of
fishes in the Amazonian waters, and in making large ichthyological
collections, for which it was very important not to miss the season of
low water, when the fishes are most easily obtained. I was, therefore,
obliged to leave this most interesting geological problem, and content
myself with examining the structure of the valley so far as it could be
seen on the river-banks and in the neighborhood of my different
collecting stations. On my return, however, when my collections were
completed, I was free to pursue this investigation, in which Major
Coutinho was as much interested as myself. We determined to select Monte
Alegre as the centre of our exploration, the serra in that region being
higher than elsewhere. As I was detained by indisposition at Manaos, for
some days, at the time we had appointed for the excursion, Major
Coutinho preceded me, and had already made one trip to the serra, with
some very interesting results, when I joined him, and we made a second
journey together.

Monte Alegre lies on a side arm of the Amazons, a little off from its
main course. This side arm, called the Rio Gurupatuba, is simply a
channel running parallel with the Amazons, and cutting through from a
higher to a lower point. Its dimensions are, however, greatly
exaggerated in all the maps thus far published, where it is usually made
to appear as a considerable northern tributary of the Amazons. The town
stands on an elevated terrace, separated from the main stream by the Rio
Gurupatuba, and by an extensive flat, consisting of numerous lakes
divided from each other by low alluvial land, and mostly connected by
narrow channels. To the west