6 Proceedings of tJie Royal Irish Academy. 
discovered in the centre of Spain, .and were described by Professor J. 
P. O'Reilly and myself, in a paper published in 1863.^' 
The beds of sulphates, which in some places are nearly sixty feet 
thick, form a line of escarpments along the left margin of the Yega, or 
alluvial plain of the river Jarama, below where it is joined by the 
Mazanares, and above where it falls itself into the Tagus. The beds of 
sulphates occupy apparently the hollow of an ancient lake, resting on 
the tertiary fresh- water limestone of the district. The river appears 
to have cut its channel along one side of the beds, and rapidly eroded 
the plain or Yega through them, leaving them only on the left side 
of the river valley. The deposits consist of thin beds of thenardite 
mixed with giauberite, and more or less discoloured with a fine greenish 
mud, and separated by bands of a similar material containing lumps 
of fibrous gypsum, and fragments of the underlying limestone. 
Some specimens of the thenardite consist of colourless compact 
masses of crystals of the anhydrous sulphate. These crystals are not 
pseudomorplis of the hydrated sulphates produced by the diying of 
masses of glauber salt, but must have formed directly as anhydrous 
sulphate from solution. The association of giauberite also proves 
that the sulphates were directly formed in the anhydrous state. 
On the beds rests a deposit of gravel, containing pebbles of the 
syenite and other rock of the Guaderama range, whence the Jarama 
issues. The deposits of sulphates we believe to be contemporaneous 
with the glacial drift of Ireland. 
These beds alford absolute physical evidence that during their 
formation the temperature of the air during part of the year was suffi- 
ciently high to raise a solution of sulphate of soda to a temperature 
exceeding 35° Cent. In order that saline solutions in lakes or shallow 
ponds should reach a temperature of 35° to 40° Cent., the temperature 
of the air should be at least 50° Cent. Temperatures even higher 
than this have been noted in Africa. Sir John Herschel found that 
the temperature of the soil in South Africa attained the great heat of 
70°-5 Cent., or 159° Eahr. On the shores of the salt lake of Bahr 
Assal, which probably at one time formed part of the Bay of Tajura, 
near the mouth of the Red Sea, Major Harris observed a temperature, 
under the shade of umbrellas and cloaks, in the beginning of June, of 
52°-24 Cent., or 126° Fahr., and conjectures that towards the end of July 
it might have reached 60° Cent., or 140° Pahr. As the temperature of 
this place, called by the Arabs the Gates of Hell, was nearly as high at 
night as in the day, there can be no doubt that thenardite and giauberite 
would be formed, if, instead of common salt, the lake contained *disodic 
sulphate. Indeed, we may be sure that among the salts which are 
now forming in the half-dried lake, are some crystals of thenardite. 
Having shown that the Spanish beds of thenardite require a high 
* Atlantis, vol. iv., p. 288 ; and Notes on the Geology and Mineralogy of the 
Spanish Provinces of Santander and Madrid, p. 139. 
