listed the following statement relating to the solubility of the sulphates of the alkaline earths in chloride of sodium : — " Sulphate of baryta is insoluble ; sulphate of strontia slowly but completely, and sulphate of lime readily soluble in an aqueous solution. The former of the two soluble salts is reprecipitated when dilute sulphuric acid is added, whereas sulphate of lime remains in solution." We have thus a simple explanation provided for the erosion of the crystals, which may have taken place either after subsidence and the readmission of the sea-water within the former dried-up area, or perhaps more simply by the infiltration of atmospheric water through the upper part of the Tertiary rock, the whole of the sedimentary formations of Egypt and Arabia being more or less charged with rock salt. Accumulation of salt water, when set up by either method, would act upon the celestine crystals, removing the soluble sulphate of strontia, leaving the Nummulites and other calcareous fossils behind. When, however, any stagnation and evaporation of the solution took place, the dissolved sulphate would be redeposited and, the mechanically included impurities being separated, the new crystals would be more compact and less liable to change than those first formed.
Besides its occurrence in detached crystals, celestine is also found in the Mokattam escarpment filling up the interior of fossil shells, especially the chambers of Nautili. Fraas gives the following analysis by Bergrath Jerzoch of a specimen of celestine taken from the inside of the shell of a Nautilus zic-zac : —
Sulphuric acid 43.87 Strontia 55.56 Lime 0.68 Loss on ignition 0.64 100.75
The conclusion drawn from this is, that it is a celestine containing a small quantity of sulphate of lime. It is more probable, however, that the lime may be there as carbonate from the presence of included shells.
3. Note on the Echinodermata, Bivalve Mollusca, and some other FossiL SPECIES from the Cretaceous Rocks of Sinai. By P. Martin Duncan, M.B. Lond., F.R.S., Sec. G.S.
The fossils collected by Mr. Bauerman from the Cretaceous strata above the red sandstone in Wady Nagh el Bader, Wady Ferran, Sidreh, and Tih during his explorations in 1868 are numerous, and some are in good condition. They present the facies so clearly stamped on the collection described by me on December 6, 1866*. The fossils which formed the groundwork for a comparison between the Sinaitic and South-eastern Arabian Echinodermata, and which were collected by the Rev. F. W. Holland, when examined critically, decided the Upper- Greensand age of the limestones in
- Quart. Journ. Geol. Soc. vol. xxiii. p. 38.
