294 GEOLOGY OF
the ground (Plate X. b). Near Sao Gabriel, and in the Uaupe"s, large masses of pure quartz rock occur, and the shining white precipices of the serras are owing, I have no doubt, to the same cause. At Pimichin, near the source of the Rio Negro, the granite contains numerous fragments of stratified sandstone rock imbedded in it (Plate IX. a) ; I did not notice this so distinctly at any other locality.
High up the river Uaupe"s there is a very curious formation All along the river-banks there are irregular fragments of rocks, with their interstices filled up with a substance that looks exactly like pitch. On examination, it is found to be a conglomerate of sand, clay, and scoria?, sometimes very hard, but often rotten and easily breaking to pieces ; its position immediately suggests the idea of its having been liquid, for the fragments of rocks appear to have sunk in it.
Coarse volcanic scoriae, with a vitreous surface, are found over a very wide area. They occur at Caripe, near Para, — above Baiao, in the Tocantins, — at the mouth of the Tapajoz, — at Villa Nova, on the Amazon, — above Barra, on the Rio Negro, and again up the Uaupe"s. A small conical hill behind the town of Santarem, at the mouth of the Tapajoz, has all the appearance of being a volcanic cone.
The neighbourhood of Para consists entirely of a coarse iron sandstone, which is probably a continuation of the rocks observed, by Mr. Gardner at Maranham and in the Province of Piauhy, and which he considered to belong to the chalk formation. Up the Tocantins we found fine crystalline stratified rocks, coarse volcanic conglomerates, and fine-grained slates. At the falls were metamorphic slates and other hard crystalline rocks ; many of these split into flat slabs, well adapted for building, or even for paving, instead of the stones now imported from Portugal into Para. In the serras of Montealegre, on the north bank of the Amazon, are a great variety of rocks, — coarse quartz conglomerates, fine crystalline sandstones, soft beds of yellow and red sandstones, and indurated clay rocks. These beds are all nearly horizontal, but are much cleft and shattered vertically ; they are alternately hard and soft, and by their unequal decay have formed the hanging stones and curious cave described in my Journal.
The general impression produced by the examination of the country is, that here we see the last stage of a process that has
