Lisbga — Pebbles on the Central Plateau of Brazil. 17 



gration of a conglomerate. This gravel generally contains 

 diamonds, especially in the valleys of the Abaete, Santo Anto- 

 nio and Somno, where they are more abundant. The material 

 of these pebbles is variable according to localities, yet sand- 

 stones belonging to diverse geological horizons, including 

 qnartzites and other metamorphic rocks, predominate in them. 

 The origin of the material of the pebbles may always be traced 

 in any one of the localities, without excepting pebbles belong- 

 ing to the oldest horizons. Qnartzites and itabirites are found 

 at Areado, about 60 miles to the east of the facetted pebble 

 beds. According to Eschwege they will be found, also in situ, 

 in the spur of the Serra da Matta da Corda, which passes by 

 the Sta. Rita coming from the basin of the Sto. Antonio to the 

 Sao Francisco, in the direction of Paraopeba. 



Climate of the region. — An important circumstance in con- 

 nection with the problem of facetted pebbles is the semi-arid 

 nature of the region. Rain is very unevenly distributed, being 

 abundant in summer and almost or quite lacking in winter. 

 The nature of the rocks also contributes to the aridness of the 

 soil. The region in summer is one of prairies which disappear 

 in winter, leaving the soil quite denuded in some places. The 

 topography of the country shows much more undulation than 

 would be expected on a high plateau, and in spite of this, there 

 are large areas where small rivulets are flowing in the rainy 

 season. For this reason much of the district is uninhabited. 



Under these conditions, it is not impossible that the high 

 parts of the ground have been influenced by wind action under 

 a semi-arid climate and that the region has experienced many 

 of the phenomena peculiar to the steppes or avant-pays of the 

 deserts. In any case it is certain that the attention of observers 

 in this country has never been turned to this subject. 



Resume. — The above notes are intended to open the way to 

 future studies, and only after such have been made will it be 

 possible to establish, or to discuss with certainty, the origin of 

 the facetted pebbles of central Brazil. Whatever theory the 

 facts may justify, this problem is one of great geologic interest. 



Hypothesis of their glacial origin. — The idea of the ice age 

 in the tropical zone of Brazil, in the Pleistocene epoch, was 

 advanced by Agassiz and has been much discussed. It was 

 attacked by Barao de Capanena in a public conference and was 

 afterward abandoned even by its author.* Thus the drift of 

 Agassiz as well as his glacial bowlders and his roches mounton- 

 nees of the vicinity of Rio are due to phenomena of decompo- 

 sition common in tropical climates. As an explanation of 



* J. C. Bianner, Jour. Geol., i, 753-772. 



Am. Joue. Sci. — Fourth Series, Vol. XXIII, No. 133.— January, 1907. 

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