90 DE. J. W. EVANS ON THE [Feb. 1 894, 



were probably granites originally, others appear to be derived from 

 gabbros or other basic rocks. 



At Uacurisal, on the western bank of the Rio Paraguay, a little 

 north of its junction with the Sao Lourengo, are found some much 

 decomposed crystalline rocks. They are more or less schistose, and 

 consist of quartz-grains often minute, small flakes of white mica, 

 and other white decomposition-products. These rocks are appa- 

 rently of igneous origin, like those at Urucum. Farther south, at 

 Dourados, are rocks of a similar character, as well as quartzites. 



I nowhere saw undoubted gneisses or crystalline schists. 

 D'Orbigny states that he found them in the Chiquitos in Eastern 

 Bolivia (see infra, p. 96). They occur widely in Eastern Brazil. 



2. Cuyaba Slates. 1 



These are highly cleaved clay-slates, apparently of great thickness, 

 though the beds are no doubt repeated by folding. They often 

 contain crystals of pyrites, usually very minute. 



The slates extend from Cuyaba north-eastward to tbe foot of the 

 Chapada plateau, where they are covered unconformably by tbe 

 Chapada Sandstones. In this direction they are much decomposed, 

 being sometimes as soft as clay. 2 They usually dip north-west at 

 40° to 55°. In some places the stratification can be distinguished 

 from the cleavage, and dips at a lower angle. 



North-west of Cuyaba the slates extend across the Rio Cuyaba, 

 and the dip increases till it is vertical, a little south of the 

 Bio Espinheiro. Farther north-north-west the dip becomes south- 

 easterly at a high angle, then it again changes to north-west ; but 

 at the Bio Jangada, where the rocks are well shown near the 

 fazenda (farm) of Dom Francisco, they dip to the south-east at 50°. 

 Beyond this point I found no exposures till I reached the remark- 

 able series of parallel ridges that run from the Bio Paraguay, 

 near Sao Luiz de Carceres (Villa Maria), north-east-by-north to the 

 east of Diamantino and west of the upper waters of the Cuyaba. 3 

 In the longitudinal valleys between these hills the slates are again 

 met with, dipping very steeply, usually north-west or south-east. 

 They underlie unconformably the Arara Limestone. 



As I have already remarked (p. 88), the Cuyaba Slates often con- 



1 Similar slates appear to occur in the States of Minas Geraes and Bahia, in 

 the region drained by the rivers Jequitinhonha and Pardo, especially in the 

 Serra de Congonha and Serra do Grao Mogor ; also at Calhao, and thence to 

 Minas Novas and the neighbourhood, (9) pp. 138-39, 151-54, 157, 163, 242- 

 43. They usually dip steeply north-west or south-east, as in Matto Grosso, 

 and are said by Hartt to resemble the gold-bearing rocks of Nova Scotia (op. 

 tit. p. 157). These slates are also found in Sergipe (op. cit. p. 405), Ceara 

 (op. cit. p. 464), and in Goyaz near Arrayas (pp. cit. p. 498) ; see note, infra, 

 p. 91. Similar slates were moreover observed by A. d'Orbigny in the Chiquitos 

 (see infra, p. 96). 



2 Hartt met with similar decomposed slates between the Capivary and Minas 

 Novas, op. supra cit. p. 154. 



3 The northern portion of these hills is known as the Serra de Tombador and 

 Serra Azul, (2) 5eme Partie, Map 9. 



