— 160 — 
nitrification into nitrate of calcium, would by reaction with the salts contained 
in the sea, produce nitrate of soda accompanied by iodine, etc. 
Mr. Hughes challenged the view that these salts are of marine origin. He 
points out that from the analyses of sea-water salts made by Forchammer and 
others (Hughes, Joe. cit., p. 160) the ratio of chlorine to sulphuric acid is prac- 
tically the same in water collected from the open sea in all parts of the world, 
viz. 100:1 1.9. The ratio in the Tafia on tlie other hand, varies greatly. The 
mean of the 2 '7 analyses from the Society's Laboratory gives 100:^6.7; 
Mackenzie's analyses 100:1 2.5; the Survey Department's 100: 33. Further 
Mr. Hughes finds no iodine or bromine. 
On the other hand, he strongly supports the organic origin of the salts, 
noting that in the aqueous solution of the tafla there are found (1) nitroge- 
nous organic matter, (2) ammonia salts, (3) nitrites, [h) nitrates. 
It is further stated that in the Nile and other rivers there is generally a 
much larger proportion of sulphates than in sea-water, and consequently the 
salt may more jirobably be derived from fluviatile or lacustrine sources. 
The discussion raised is one of great interest, and the point has been consi- 
dered by Mr. Lucas [Natural Soda Deposits in Egypt, p. 20). He admits for the 
sake of argument that the sulphates may.be in higher ratio to the chlorine 
than in sea-water, though in general few samj)les have been examined. In the 
special case of the Wadi Natrun, Mr. Lucas submits the possibility that the 
region was never completely cut off from the sea, but only partly shut olTin 
such a manner that concentration of the water with deposition of the more 
insoluble salt could take place, the more soluble salts meanwhile remaining in 
solution, and that eventually this concentrated mother-liquor escaped back 
into the sea or was replaced by a fresh influx of sea-water. In this way, calcium 
sulphate could be deposited in a ratio to sodium chloride greater than that in 
which it exists in sea-water. He further indicates the importance of past rain- 
falls in disturbing the proportions as between chlorides and sulphates, the 
more soluble sodium chloride, for instance, being removed, and the more inso- 
luble calcium sulphate being left behind. 
The present writer must confess that the geological facts had strongly 
biassed him in favour of an origin for the salts under conditions where muds 
were beinglaid down in deep sea-water. This might be very difl'erently composed 
