322 Proceedings of the Royal Physical Society. 
that. The same district may, under changing geographical 
conditions, pass in course of time from one hyegrometric 
extreme to the other. Under the maximum rainfall, and 
especially if this rainfall is fairly distributed throughout the 
year, vegetation will mostly flourish; and, with an abundant 
vegetation, will occur also more or less abundance of animal 
life. Rivers flowing into the lakes will (as the Jordan does 
into the Dead Sea, for example) transport more or less of 
this organic matter into the lake, where the concentration of 
salts is in progress. Under these conditions, as we shall 
presently see, results very different in their nature from those 
that obtain under more arid conditions will obtain. If, on 
the other hand, the nature of the precipitation is such that, 
if the maximum, it falls in large quantities at a time, and 
without regular recurrence, or if the rainfall is nearer the 
minimum above-mentioned, then vegetation (except of the 
scrubbiest kind) cannot maintain its existence. The soil 
becomes bare, and is frequently drifted from place to place 
by the wind, animals disappear, and the place becomes a 
desert. Under these circumstances, little or no organic 
matter finds its way into the lake, concentration goes on 
apace, and the chemical reactions arising from the deposition 
of the various salts carried in by rivers differ in most 
important respects from those in the alternative case just 
considered. 
As bearing upon the nature of the changes that ensue 
under either set of conditions, I can hardly do better than 
abstract, as nearly as possible in the author’s own words, the 
substance of an address to the Geologists’ Association, by 
W. H. Hudleston, F.R.S., “On the Geological History of Iron 
Ores.” ! [Tron dissolved in the first instance from the rocks of 
the land by the action of the humic acids and their ultimate 
term carbonic acid, eventually become diffused through 
the waters of the rivers and lakes. Those parts of the solu- 
tion which are near the surface of the water seize upon an 
additional molecule of oxygen, thereby losing their carbonic 
acid and becoming transformed into ferric oxide, which 
immediately combines with water to form ferric hydrate, in 
1 Proc, Geol. Assoc., vol, xi., No, 3, pp. 104-144. 
