422 



E. C. HARDER AND R. T. CHAMBERLIN 



the streams began dissecting the old nearly base-leveled plain, 

 first cutting through the surface decomposition products and associ- 

 ated fluviatile deposits, and then attacking the underlying quartz- 

 ite. As the chapada deposits were being removed in this erosive 

 process, the softer and lighter materials were being worn finer and 

 washed away, while the heavier diamonds remained behind and 

 became further concentrated in the stream gravels of the immediate 



Fig. 24. — Diamond mining at Cadette's mine. The clay and materials of lesser 

 specific gravity are washed away by running water while the diamonds and other 

 heavy pebbles remain behind and become concentrated. 



vicinity. At several stages during this erosion there were periods 

 when the downward cutting of the streams was checked and they 

 began to widen their valley bottoms and deposit material over 

 them. Then active cutting began again, leaving gravel-covered 

 terraces on the valley slopes. Much material from these several 

 gravel terraces as well as from the diamondif erous conglomerate of 

 high-level chapadas has since been washed down the present steep 



