44 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 9, 



listed tlie 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 dis- 

 solved 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 filhng up the interior of fossil shells, 

 especially the chambers of Nautili. Fraas gives the following ana- 

 lysis 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 Echinodeemata, Bivalve Molltjsca, and some other 

 EossiL SPECIES /rom the Cretaceotjs Bocks o/ Sinai. By P. Martin 

 Duncan, M.B. Lond., E.B.S., Sec. G.S. 



The fossils collected by Mr. Bauerman from the Cretaceous strata 

 above the red sandstone in Wady Kagh el Bader, Wady Eerran, 

 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 be- 

 tween the Sinaitic and South-eastern Arabian Echinodermata, and 

 which were collected by the Bev. E. W. Holland, when examined 

 critically, decided the tipper- Greensand age of the limestones in 

 * Quart. Journ. Greol. Soc. vol. xxiii. p. 38. 



