18 



the early release of flood water from the lake, the high flood 

 levels of former years are not likely to be repeated. The 

 highest flood level recorded is 8J feet above mean sea level, a 

 height which signifies that the volume of water in the lake has 

 been increased to at least four times the normal ; that to the 

 one volume which represents the normal have been added three 

 volumes of fresh (sweet) water. As the specific gravity of the 

 lake is not more than 1,030 under ordinary conditions the 

 addition of three volumes of fresh water will reduce it approxi- 

 mately to 1,008, a reduction sufficient to prove fatal to many 

 animals which might otherwise find suitable life-conditions 

 within the bounds of the lake, Placima placenta in particular, 

 a mollusc living to-day in the adjoining sea, and partial to 

 muddy bottom and land-locked bays. 



Nature of the lake bottom. — Fine quartz sand and an 

 extremely smooth dark mud, greyish black to full black in 

 colour, are the two main constituents of the bottom. 



Above high-tide level the shore is very generally composed 

 of clean yellow sand, but except for the bar channel and the 

 shoals adjacent, subject to a heavy daily tide-scour, the bottom 

 consists either of fine mud or of sand and mud in varying 

 proportions. East of Pulicat, in Karimanal inlet, in the 

 shallower water between Karimanal and the Annamaiaicheri 

 side and generally along the lake shore for varying distance 

 from high-tide level, sand occurs mingled with mud. Pure 

 mud, soft and oozy, forms the bottom everywhere else, that is, in 

 all depths greater than one foot below mean sea level, except in 

 the neighbourhood of the bar. 



In the muddy sands in the vicinity of Pulicat, as also in 

 Karimanal inlet, considerable numbers of bivalves are found, 

 usually in distinct beds ; as a consequence, the lake bottom in 

 these sections contains in places a considerable admixture of 

 dead shells. In the mud deposit of the deeps, shells of any 

 kind are seldom seen — no molluscs live in this mud when it 

 is of any depth ; as it is now, it forms one of the most barren 

 and valueless bottoms imaginable, the only life supported being 

 a scum of diatoms particularly abundant wherever the water is 

 very shallow. 



Salinity and temperature. — Both density and temperature 

 are high as may be inferred from the physical features of the 

 lake. The former is influenced by the enormous area of shallow 

 water exposed to rapid evaporation, by the slight influence of 

 the tidal wave away from the immediate vicinity of the bar, 

 and by proximity at the north end to salt marshes and salt 

 impregnated strata. 



