18 Lisboa — Pebbles on the Central Plateau of Brazil. 



facetted pebbles, the hypothesis of the Pleistocene glaciation is, 

 therefore, out of the question. 



There remains the hypothesis of a Permo-Carboniferous 

 glacial epoch. Permian rocks exist in southern Brazil, but in 

 the central region they have not hitherto been reported in the 

 extensive basin of the upper Sao Francisco. As far as the 

 geology of the region is known, there is no positive evidence 

 in contradiction to this hypothesis, and any facts supporting it 

 would be of great geological importance. 



Another problem of no less importance is that of glaciation 

 at the close of the Paleozoic era. When Waagen wrote on 

 the subject of glaciation in the southern hemisphere, in the 

 Permo-Carboniferous epoch, he referred to this part of Amer- 

 ica. His observations drew forth a letter from Prof. Derby in 

 which certain pebbles and bowlders of Itti, S. Panlo, were 

 referred to glacial action of that period, and it was said that 

 the lack of positive evidence of glacial phenomena was due to 

 a lack of knowledge of the country.* The subject has since 

 been left on the same footing. The absence of striae is a nega- 

 tive fact of great importance, but until the present time no 

 one has ascertained whether local conditions are not unfavor- 

 able to their preservation. 



Hypothesis of their ceolian origin. — The climatic conditions 

 of the district are such that the hypothesis of wind action is 

 naturally suggested, but here also the author finds not a few 

 difficulties. 



Rains on the extensive central Brazilian plateau vary greatly, 

 not only from year to year but also locally. On the whole the 

 region may be considered semi-arid and even arid in some 

 parts. 



The climate of central Brazil has been but little studied. 

 In one of his recent writings, Professor Derbyf calls attention 

 to this fact and observes that the meteorological data gathered 

 on the coast are often erroneously applied to the arid regions 

 of northern Brazil. He compares some zones of the central 

 plateau of Brazil;}; to the great American desert. 



Certain conditions in the occurrence oj the facetted pebbles 

 seem to exclude the presumption of their being of recent date. 

 This point may be easily ascertained with further observations. 

 We are considering a large surface of country approximately 

 with the same conditions of climate, topography and geography 



* Waagen and Derby, Mittheilnng eines Briefes von Herrn A. Derby liber 

 Spuren einer carbonen Eiszeit in Sudamerika. Neues Jahrbucb fur Mineral. 

 1888, II Band, Brietliche Mittheil., p. 172. 



f O. A. Derby, reigmen das chuvas nas regioes' das seccas. Jornal do 

 Commercio, Rio de Janeiro, 34 de Marco, 1906. 



% With a medium rainfall below one meter and evaporation more than 

 twice as great as precipitation. 



