It was first necessary to devise an a rbitrary measure of 

 what constitutes pollution of the bottom deposits as determined by 

 the kinds and abundance of organisms. From a study of many samples 

 it was decided that, for mud bottom, less than 100 Tubificidae and 

 more than 100 Hexagenia per square meter could be considered as 

 indicative of clean conditions. More than 99 Tubificidae and less 

 than 101 Hexagenia per square meter was considered to be indicative 

 of a polluted bottom. It seemed desirable, also, to recognize dif- 

 ferent degrees or zones of pollution, such as light, moderate, and 

 heavy pollution. These degrees or zones were arbitrarily determined 

 on the basis of the number of Tubificidae as follows: light 

 pollution, 100-999; moderate pollution, 1000-5000j heavy pollution, 

 more than 5000 per square meter. It was found that in areas where 

 the number of Tubificidae indicated heavy pollution, Hexagenia was 

 usually entirely absent; but in the moderate and light zones the 

 number of Hexagenia varied between and 100, and did not show a close 

 correlation with the number of Tubificidae. For that reason the 

 number of Tubificidae alone was used in determining the limits between 

 pollutional zones. A few stations on mud bottom showed very few of 

 either of the two index organisms. In such cases, the number of 

 Tubificidae was used as the criterion with regard to pollution. 



On other than mud bottoms it was found, as previously stated, 

 that Hexagenia had little or no index value, because even in the 

 absence of organic debris it was rare or altogether wanting. The 

 Tubificidae could be used as an index of pollution regardless of the 

 substratum, because they appear to thrive wherever there is an accumu- 

 lation of organic debris. 



As an aid in the interpretation of results, a note regarding 

 the type of bottom has been inserted in many of the tables of this 

 paper. To conserve space, a system of symbols was used, as follows: 



M = mud 



S = s and 

 MS = principally mud, some sand 

 SM = principally sand, some mud 



G = gravel 



C = hard clay 



R = bed rock or boulders 

 CS = hard clay overlain by sand 



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