Lisboa — Pebbles on the Central Plateau of Brazil. 19 



throughout, and consequently the effects of wind-blown sand 

 would be general. At present it may be presumed that facet- 

 ted pebbles are of exceptional occurrence in the country under 

 consideration. In the hypothesis even of action restricted to 

 a certain locality, the aeolian effect would not be limited to a 

 few pebbles but would extend to them all, leaving at the same 

 time an unquestionable proof on the surface of the surround- 

 ing rocks, and it is very singular that, if these effects exist, 

 they should not have been noticed. 



The recently published observations of Prof. J. C. Branner* 

 on the formation of the reefs of the coast show that the exist- 

 ence of an arid climate in this part of the continent should 

 date from Cretaceous times and that this aridness must con- 

 tinue to occur as long as the present geographic conditions 

 remain. This allows the possibility of referring the formation 

 of facetted pebbles to a period anterior to ours, perhaps as far 

 back as the Cretaceous, and possibly under a still more arid 

 climate. 



EXPLANATION OF FIGUEES. 



Figures 1 and la, 2 and 2a, 3 and 3a, 4 and 4a, 5 and 5a. — Facetted pebbles 

 of pyramidal type, elongated shape. Seen from the faces opposite 

 to the base and from the side. 



Figures 7 and 7a. — Flattened form, la shows the base of pebble No. 7. 

 This face is identical in all the samples except number 3. 



Figures 6, 8, 9. — Facetted pebbles of diverse shapes. 



*The stone reefs of Brazil, Bulletin of the Museum of Comparative 

 Zoology, Harvard College, xliv, Geological Series, vol. vii, May 1904 ; also, 

 Bull. Geol. Soc. Amer., xvi, 5 and 12, Feb. 1905. 



