1879.] 1^7 [Derby. 



Situated on the margin of the varzea, and about two leagues to the west- 

 ward of the village of Monte Alegre, there is an isolated group of moun- 

 tains, consisting of numerous, small monoclinal ridges, separated one from 

 another, and disposed in an ellipse around a central plain, of which the 

 elevation is a few feet at most above the level of the Amazonas. The 

 major axis of the ellipse is some ten or twelve miles long, and lies in the 

 direction of E.-W. The principal mountain, called Serra de Tajuri, is about 

 350 metres high, and is situated on the north-east side of the ellipse ; from 

 Tajuri, a curved line of low ridges extends to the second serra in size, that 

 of Erere, which is on the southern side, and has an elevation of 250 metres ; 

 then come the smaller serras of Aroxi, Maxira, Paraizo, Juliao and Uru- 

 cury, the last being placed at the western end of the ellipse ; between this 

 and Tajuri there is a considerable number of low serras, without names, 

 which have never been explored. All of these serras present an abrupt 

 slope towards the central plain, and a gentle slope, following the inclination 

 of the strata, on the opposite side. This inclination, which varies from 10° 

 to 20°, is N. N. E. in Tajuri, E. in the ridges between Tajuri and Erere, 

 S. in this last, and W. in Urucury. This variation in dip proves that this 

 group of mountains is only the remnant of a great anticlinal, of which the 

 central, and by far the larger, part has been denuded away. This opinion, 

 respecting the structure of the region, is supported by the structure of the 

 low Serra de Paituna, which is situated outside of the ellipse, some three 

 or four miles to the south of the Serra de Erere, with which it is parallel. 

 As was to be expected from its position in relation to the other mountains, 

 Paituna was found to be a synclinal ridge. It is possible that to the north- 

 ward of Tajuri there are other synclinal ridges, similar to Paituna. 



There have not yet been definitely recognized in other parts of the val- 

 ley, any elevations, corresponding in age and structure to those of Erere. 

 I have reasons, however, for believing that, in the vicinity of Obydos, the 

 Serra de Curumu and perhaps that of Cunury may belong to the same sys- 

 tem. Near the margin of the metamorphic region, on the Guiana side at 

 least, the Palaeozoic beds are gently inclined, at an angle of 5°-10° ; but in 

 general these beds appear to lie horizontally. 



The first member of this Palaeozoic series of the Amazonas is the Upper 

 Silurian. The rocks of this age appear on the Gniana side, in a belt of a 

 few miles in width, which extends in the direction east-west for a consid- 

 erable distance, if not along the whole southern margin of the metamorphic 

 region of Guiana. They have been recognized on the Trombetas, Curua 

 and Maecuru, and from specimens brought by Sr. Ferreira Penna, from the 

 Maraca, a small river which empties nearly opposite the western end of 

 Marajo, I judge that they extend eastward nearly to the Atlantic. 



These rocks have been best studied on the Trombetas. They there appear 

 in a belt four or five miles wide, forming the first and part of the second 

 cachoeira. They were also found, well exposed, in a hill of some 100 

 metres of elevation, called Oiteiro do Cachorro, situated on the right bank 

 of the river of the same name, a little above its confluence with the Trom- 



