In the neighborhood of Almeyrim, at a short distance from the northern bank of the river, and nearly parallel with its course, there rises a line of low hills, interrupted here and there, but extending in evident connection from Almeyrim through the region of Monte Alégre to the heights of Obydos. 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.[99] 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 Alégre 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 took a second journey together. Monte Alégre 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 of the town this terrace sinks abruptly to a wide sandy plain called the Campos, covered with a low forest-growth, and bordered on its farther limit by the picturesque serra of Ereré. The form of this mountain is so abrupt, its rise from the plains so bold and sudden, that it seems more than twice its real height. Judging by the