Yol. 50.] GEOLOGY OF MATTO GEOSSO. 103 



latter. I have not visited the workings, which have to a large 

 extent been discontinued since the abolition of slavery, as the work 

 is very unhealthy. The following details of the deposits are sum- 

 marized from Castelnau, (2), l hTe Partie, vol. ii. pp. 319-3-13: — 



4. Black argillaceous earth. 



3. Gorgalho. Small sandstone and quartz-pebbles cemented, by 



yellow clay. 

 2. Cascalho. Larger pebbles witbout cement. 

 1 . Eed argillaceous sbales (probably Matto Shales). 



In No. 2 are found, in addition to the constituents of No. 3, a 

 black or mottled quartz, a line hone-like sandstone (probably derived 

 from the Bizama Sandstone), a violet sandstone, and diamonds. The 

 three former are regarded as indicators of diamonds. I found 

 similar pebbles in the Paraguay and its tributaries at and above 

 Barra dos Bugres, but saw no diamonds. 1 Castelnau states that 

 sandstones occur in the vicinity of the workings ; from these he 

 supposes the diamonds to have been originally derived. 



Iron occurs in abundance, especially at Uruciim. 



Manganese is also found at Urucum, where it occurs in thick 

 deposits, which appear as a band of black cliffs halfway up the 

 mountain. 



Copper (on the Bio Jauru), lead, and other metals are stated to 

 occur, (5) vol. i. p. 148. 



In conclusion, I may mention that specimens' and thin sections 

 from the principal localities which I visited are deposited at the 

 British Museum (Natural History), South Kensington. 



EXPLANATION OF PLATE VIII. 



Geological Map of a Portion of South-eastern Matto Grosso. 



Kofe. — The alluvial region shown in the map is a vast stretch of swampy low- 

 land, which is under water during great part of the year. For ' Livra- 

 menta ' read ' Livramento.' The position of Arara is on the limestone- 

 outcrop south of the Rio Oiripuru, and west of Livramento. 



entirely so. The matrix is ferruginous and highly magnetic. An analysis by 



Mr. G. T. Holloway gave the following results: — 



Per cent. ^ 



Silica 95-970 



Iron 2690 corresponding to ] „h 1ft 



Magnetite ] <mu 



Alumina -423 j-=100'665 



Lime -460 



Magnesia -102 



Manganese traces 



Phosphoric acid traces 



Mr. Derby tells me that it resembles a weathered specimen of a pyritiferous 

 quartz-veinstone from Minas Geraes. The pyrites appears to have been removed, 

 and additional silica deposited in the cavities left. He thinks that such a rock 

 may be auriferous, but is not likely to contain diamonds. 



1 A small crystal of topaz with rounded edges was found there. 



