J. J. Griffith 377 



Table IV. Composition of Sediment') from Rivers and Mine Leats. 

 Percentages in air-dried samples. 



(b) Tlie clear waters of the rivers are fairly free from poisonous 

 ingredients. But the suspended matters in both leat and river waters 

 often contain relatively large quantities of lead and zinc. 



(c) Occasionally some of the streams contain considerable quantities 

 of the poisonous metals in the form of soluble compounds (see note, 

 Table II). This happens more especially when the rainfall is heavy 

 after a long period of drought. Some of the mine heaps are situated on 

 peaty land. The sourness of the surrounding land may assist the process 

 of solution of the lead in the sediment washed on to its surface from the 

 heaps. Consequently surface drainage from such land may contribute 

 largely to the pollution of the adjoining river with soluble lead com- 

 pounds at the time of the first heavy rain after a period of drought. 

 This may be the reason why, according to the evidence of some farmers, 

 vegetation on contaminated soils is more poisonous to stock after the 

 first flood subsequent to drought than at any other time. 



(d) The mechanical composition of the mine "sand" and of the 

 sediment of polluted rivers is such as may lead to unfavourable changes 

 in the physical properties of light soils. 



(e) The sediment in the leats is much finer than that found in the 

 river beds. It approaches the mine slime in mechanical composition. 

 It is found, however, to contain a much higher percentage of the poison- 

 ous metals. 



