SEA-FISHERIES LABORATORY. 359 



smells badly. I should compare this deposit with the 

 sludge in a sewage purification tank — in kind only; 

 of course it is not nearly so foul. No doubt it 

 has become thoroughly permeated with sewage 

 which is there undergoing putrefactive purification, 

 with an accompanying deposition of the amorphous 

 black substance which forms the sludge of a 

 septic tank, and with the liberation of sulphuretted 

 hydrogen, and perhaps marsh gas. Both sewage paper 

 and faecal matter were very much in evidence in this 

 stream and on its banks. Of course, this outfall and area 

 are at some distance from the larger mussel beds, but it 

 must contribute to the pollution of the rivers.* 



In addition to these sources of contamination there 

 must be others further up the Conway river. The sewage 

 of some of the villages in the neighbourhood, such as 

 Llanrwst, Grlan Conway and Bettws-y-Coed, must be 

 discharged into the upper reaches of the Conway, and no 

 doubt contribute notably to the pollution of the latter. 



The sewers I have referred to all convey crude sewage 

 and discharge continuously into the river. It is true that 

 tidal flap valves are fixed at their ends, but I question 

 whether these fixtures can generally act in preventing the 

 flow of liquid from the pipes when the latter are covered 

 by the flood tide. In some cases the flaps were certainly 

 choked by debris of various kinds and could not have 

 closed. At any rate, the sewers discharge through the 

 period of ebb tide and for a considerable part of the flood, 

 since their outfalls are mostly well above low-water mark. 

 It is especially near the time of low water, when the 

 volume of the river is small, that the latter becomes 



* It will be seen from the Chart that the short stream conveying 

 this sewage into the Conway opens into the latter immediately above 

 the bridge. Mussels are taken from just this part of the river — in 

 fact almost opposite the mouth of the stream referred to. 



