16 Lisboa — Pebbles on the Central Plateau of Brazil. 



Materials of the pebbles. — All the samples examined were 

 of quartzite or metamorphosed sandstone, older than the dark 

 shales and sandstones that constitute the rocks of the plateau 

 where the specimens were found. These older rocks have been 

 found on the highest parts of the plateau, forming- the mate- 

 rial of the superficial gravel, and often also in isolated blocks. 

 They have been considered as formed from the disintegration 

 of the conglomerates of horizontal shales and sandstones.* 

 These pebbles are considered to be contemporaneous with the 

 formation of the plateau sediments. 



Geology of the region. — The following geological notes of 

 the district may serve as preliminary information until more 

 detailed observations are made with a view to determining the 

 origin of the facetted pebbles. 



As a rule, the rocks of the lower parts of the Borrachudo 

 and Abaete valley belong to two groups. The lower group 

 consists of argillaceous shales and hard, bluish sandstone that 

 are considered to be of Lower Paleozoic age. These rocks 

 come to the surface in the deepest parts of the valleys. On 

 them rests another group of rocks likewise composed of hard, 

 bluish sandstone and argillaceous shale which is hardly dis- 

 tinguished from that of the preceding group except for its 

 constant approximate horizontality. The line of contact be- 

 tween these two groups may be observed near the localities of 

 the facetted pebbles not far from Bio Abaete. 



This last group forms the plateau of the region, rising 700 

 to 800 meters above sea level. As to the age of this group it 

 is impossible to make a definite assertion. Fossil wood frag- 

 ments, which have lately been identified as angiospermsf and 

 which are referred to by Liais as belonging to the sandstone 

 formation;}; of the sources of the Indaya, seem to indicate the 

 existence in the district of rocks of the Upper Jurassic, or 

 more probably, of Cretaceous age. The occurrence of these 

 angiosperms has not yet been described in detail, nor are there 

 any positive notes on their relation to the different rocks of the 

 region. It is not safe, therefore, to consider the rocks of the 

 Borrachudo and Abaete plateaux as belonging to the Upper 

 Mesozoic age, before further observations have been made. 



Mention should here be made of the common occurrence in 

 the district of superficial deposits of gravel, not only at the 

 bottoms of valleys but also on high parts of the plateaux. The 

 high gravels have been considered as the result of the disinte- 



* 0. A. Derby, Reconliecimento geologico dos valles do Rio das Yelhas e 

 Alto S. Francisco, p. 29. 



f O. A. Derby, Nota sobre a geologia e Paleontologia de Matto-Grosso. 

 Archivos do Muzeu National, p. 68. 



\ Cited by O. A. Derby in Paleontologia de Matto Grosso ; Liais, Geologie 

 du Br<5sil, p. 210. 



