10 Lisboa — Pebbles on the Central Plateau of Brazil. 



considered to be a proof of local glacial action. Later, when 

 Behrendt's theory was displaced by the theory of the action 

 of sand blown by wind, the same pebbles were taken as a 

 proof of existence of arid wastes or steppes in different geo- 

 logical periods. 



Prof. Johnson* observes that in treatingjof existing deserts 

 the glacial origin of facetted pebbles musT be abandoned, so 

 that,, in view of the evidence of the Permian facetted pebbles 

 and of the glacial pebbles of the south pole, we are confronted 

 by two theories that demand essentially different geological 

 conditions. It does not seem likely that two such different 

 causes as glaciers and wind-blown sand would produce exactly 

 the same type of facetted pebbles. To a certain extent, the 

 existence of strise would offer a point of reference for this 

 determination, but the difficulty of proving their formation 

 contemporary with the formation of the facets, and their 

 complete absence in pebbles of undoubted glacial origin, show 

 that other characteristics most be found to make it possible to 

 discriminate between the two distinct types. This discrimina- 

 tion would be established if unquestionable facts confirmed 

 the co-existence of the two causes in the formation of the 

 pebbles. 



From these considerations it is clear that only after the prob- 

 lem is completely solved will the occurrence of facetted peb- 

 bles form a secure criterion for the determination of glacial or 

 seolian phenomena. A careful study of each occurrence seems 

 now indispensable for the discovery of the causes of formation 

 of these much discussed pebbles. From this point of view, 

 the notes taken by the author are very defective. At the time 

 of the discovery of these pebbles, his time would not permit a 

 detailed study of them, and being unacquainted with the liter- 

 ature of the subject, it was not until some time later that he 

 gave them the attention they deserved. The present notice, 

 therefore, is only a description of the facetted pebbles accom- 

 panied by a general outline of the geology of the district in 

 which they occur. The author does not intend to advance an 

 opinion as to their origin before studying the local geological 

 conditions. 



The locality. — During the months of September and Octo- 

 ber, 1905, I visited the central plateau of Brazil, for the pur- 

 pose of studying the diamond deposits on the left side of the 

 upper basin of the S. Francisco River, a district about TOO 

 miles from the Atlantic coast. There was but little time for 

 geological observations outside the direct object of my trip, 

 except a rapid survey of the roadsides or short excursions 



* Zur Entstehmig der Facettengesteine. Centralblatt. f. Min., Geol. u. 

 Pal., 1903, n. 19, p. 593. 



