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E. C. HARDER AND R. T. CHAMBERLIN 



channels in the old surface, are found deposits of residual clay and 

 gravel, more or less imperfectly cemented. This material is all 

 thoroughly weathered and of a residual nature. The pebbles in 

 the conglomerate are mostly of quartzite, but with a sprinkling of 

 pebbles of iron formation, schist, quartz, basic igneous rock, and 

 diamonds. With the conglomerate is associated a considerable 

 amount of sand and of clean white to pinkish clay, mostly kaolin, 

 with some bauxite. This material for the most part represents 



Fig. 9. — A chapada, or table-land, forming the summit of the Backbone Range near 

 Diamantina. The diamond-bearing conglomerates are accumulations of residual 

 gravels collected in depressions or old stream channels on the peneplained surface. 



the final products of the rock weathering, and its presence perched 

 upon the smooth crest of the Serra do Espinhago above the rest of 

 the region would suggest that it is to be connected with the base- 

 level stage in the history of the region. This is the celebrated 

 diamond-bearing conglomerate which has brought fame and wealth 

 to the district, and which would be interpreted in physiographic 

 studies as the high-level gravels which mark the peneplain stage. 

 Subsequently, during later erosion cycles, some of the conglomerate 



