Although the forests cover generally the whole length and breadth of the Amazonian Valley, there are here and there, on the higher ground, open dry plains with scanty vegetation,--the ground in the water-courses or gullies, formed of clay, being baked by the heat of the sun into slate-like masses. One of these spots we now reach. The most prominent plants of this sandy or clayey region are clusters of cacti and curua palms--a kind of stemless, low palm, with broad leaves springing, vase-like, from the ground. Here also grow wild pineapples; and in broad sunlight numerous humming-birds delight to sport and feed upon the blossoms of the various plants which find no room to bloom in the darker shades of the forest. GEOLOGY OF THE AMAZONIAN VALLEY. Professor Agassiz remarks that no formation--known to geologists-- resembling that of the Amazon exists on the face of the earth. Its extent is stupendous. It stretches from the Atlantic shore through the whole width of Brazil into Peru, to the very foot of the Andes--one vast extent of red sandstone, capped by a yellow-ochred clay; not only along the banks of the main river, but forming the sides of those of its tributaries, to their far-off sources, probably over the whole basin of the Paraguay and the Rio de la Plata. How are these vast deposits formed? is the question. The easiest answer, he observes, and the one which most readily suggests itself, is that of a submersion of the continent at successive periods--to allow the accumulation of these materials--and its subsequent elevation. This explanation is rejected, for the simple reason that the deposits show no signs whatever of a marine origin. No sea-shells, or remains of any marine animal, have as yet been found throughout their whole extent--over a region several thousands of miles in length, and from five to seven hundred miles in width. It is evident, he considers, that this basin was a fresh-water basin, these deposits fresh-water deposits. It is true that calcareous layers thickly studded with shells have been found interspersed with the clay; but though supposed to be marine fossils, he recognised them for what they really are--fresh-water shells of the family of the Naiades. As their resemblance is very remarkable, the mistake as to their true zoological character is natural: indeed, many travellers have confounded some fresh-water fishes from the Upper Amazon of the genus of Pterophyllum with the marine genus Platax. He considers that the immense glacier which probably existed at the same time that ice, thousands of feet thick, covered the centre of Europe, must have been formed in this valley, and then, ploughing its bottom over and over again, and grinding all the materials beneath it into a fine powder, must ultimately have forced its way through the colossal sea-wall which it had built up eastward into the Atlantic. A DAY AND NIGHT ON THE AMAZON, WITH THEIR SIGHTS AND SOUNDS.