326 Proceedings of the Royal Physical Society. 
present in the atmosphere is below the average amount. 
Where vegetable matter is carried into the lakes in lesser 
quantity, changes of a different nature occur; but with these 
we are not at present specially concerned. 
In regard to the effect of these concentrated solutions of 
sulphate of lime and other salts upon the vegetable matter 
itself, we have as yet no absolutely certain data upon which 
to base any satisfactory conclusions. But we are enabled, it 
seems to me, to form some general idea of the nature of the 
chemical processes which go on under these circumstances 
by studying the field evidence, and by noting the facts 
common to a large number of cases where deposits of 
gypsum occur. Briefly summarised, the facts gleaned from 
these sources of information are these:—There is a very 
common, one might almost say, universal, association of rock- 
salt and gypsum with two types of strata. In one of these 
the associated strata are coloured by ferric oxide, and in that 
case not a trace of any kind of vegetable matter, or of 
bituminoids derived from it, is to be found. In the other 
case, where the gypsum is associated with rocks which are 
not coloured with ferric oxide, traces of vegetable matter are 
occasionally, but rarely, present, and then in a more or less 
bituminised form; but the rocks themselves are often, so to 
speak, saturated with one or other of the hydrocarbon 
compounds, amongst which bituminoids usually occupy a 
conspicuous place. There appears to be some reason to 
believe that what happens in such cases is that a cross 
reaction takes place between the decomposing vegetable 
matter and the solutions of sulphate of lime. Precipitation 
of lime in the form of carbonate is one result, and the other 
would appear to be the rapid maceration of the fermenting 
vegetable matter, and the conversion of that macerated 
vegetable matter into one or other of the bituminoids, in 
which form it is diffused throughout the rocks forming at 
the time at the bottom of the lake. I am disposed to regard 
this as the origin of the bitumen found in the Dead Sea, and 
in so many other inland lakes into which vegetable matter 
is drifted. Even in the case of the Pitch Lake of Trinidad, 
it seems to me that the uprise of water containing sulphate 
