294 



GEOLOGY OF 



the ground (Plate X. h). Near Sao Gabjiel, and in the 

 Uaupes, 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. d) ; I did 

 not notice this so distinctly at any other locality. 



High up the river Uaupes 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 scoriae, 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 v/ide area. They occur at Caripe, near Para, — 

 above Baiao, in the Tocanti'ns, — at the mouth of the Tapajoz, 

 — at Villa Nova, on the Amazon, — above Barra, on the Rio 

 Negro, and again up the Uaupes. 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 Tocanti'ns we fo^-ind fine crystalline 

 stratified rocks, coarse volcanic conglomo .ites, 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 

 in lurated clay rocks. These beds are all nearly horizontal, 

 buc 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 



