Vol. 50.] GEOLOGY OF MATTO GEOSSO. 95 



of these shales and occasional interstrafcified grit-beds are seen at 

 intervals. A steady dip of about 15° north-north-west soon sets in, 

 and continues as far as the neighbourhood of Tres Barras. 1 Here 

 the Bio Sant' Anna joins the Paraguay, which has just received the 

 Rio Erumada. I followed the shales for a short distance up tho 

 Rio Sant' Anna (where they dipped at 10° north-north-west), and 

 also up the Ilio dos Bugres and its tributary tho Rio Brazinho. 

 Along the two last-named rivers the shales were more horizontally 

 bedded. 



I was informed by reliable Brazilian traders that similar shales 

 are found on the Rio Sepituba, and red argillaceous shales are de- 

 scribed by Castelnau as occurring near Diamantino, (2) l fere Bartie, 

 vol. ii. p. 323. 



I travelled by land about 100 kilometres (or 62 miles) north-west 

 of Barra dos Bugres. No exposure of the shales is met with in the 

 forest, but the nature of the soil and vegetation indicates their con- 

 tinuance till the hills of Tapirapuam are approached. The soil then 

 becomes ferruginous, and the trees smaller and fewer, till a tract of 

 peaty grass-land is reached : beyond this are irregular hills of an 

 olivine-basalt, which weathers into rounded elevations. Behind 

 and above is a line of flat-topped hills, apparently the edge of a 

 sandstone-plateau, which I was unable to visit. Farther north 

 runs a second escarpment, the Serra dos Parecis. 



V. Devonian Rocks. 

 6. Chapada Sandstones. 2 



These sandstones appear to extend over the whole of the great 

 plateau of Matto Grosso, although in some places covered by later 

 beds. They have a slight dip towards the north. On the south, 

 near Cuyaba, they terminate in a lofty escarpment nearly 600 

 metres (1950 feet) above the undulating slate-plain. The lower half 

 of the escarpment-face consists of a steep slope, composed of the 

 highly-inclined slates. Above are vertical cliffs of red sandstone, 

 conglomeratic below, which higher up again pass into another slope. 

 The cliffs are undercut by the disintegration of the slates, which yield 

 much more readily than the other rocks to the action of eroding 

 agents. Large masses of the sandstone appear to break off from 

 time to time, owing to the removal of their support. As has been 

 remarked by Chandless (5), from the edge of the plateau the wide 

 slate-plain looks like a sea, and the escarpment, with its inlets 

 and promontories, has a curious resemblance to a coast-line. It is 



1 Above this point the Paraguay is an insignificant stream, it cannot be navi- 

 gated in large canoes, and according to some authors is improperly called the 

 Paraguay, (5) vol. i. pp. 107, 121. 



2 Sandstones with the same characters and probably of the same age are 

 found on either side of the valley of the Rio Sao Francisco, where they form 

 chapadas like that near Cuyaba, (9) pp. 32, 310, (IS) p. 7. Similar sandstones 

 also occur in Sao Paulo and the other Southern Brazilian States, (11) p. 2o4, 

 (lo) p. 8, as well as in the Chiouitos (see infra, p. i)G). 



