164 



August 5, 1846. 



D. H. Storer, M. D., Vice President, in the Chair. 



Dr. John Bacon, Jr. read notices of Polythalamia, in the 

 sand of Sahara Desert. 



Under the microscope the sand is seen to consist mainly of 

 irregular quartzose grains, of a reddish yellow color, and a pretty 

 uniform size, ranging between 0.01 and 0.03 of an inch. These 

 grains are more or less rounded on the angles and edges. A 

 few minute quartz crystals are also met with, presenting the usual 

 six-sided prism, with pyramidal terminations, rounded in the same 

 manner. It is easily understood how the wind raises in clouds 

 and conveys to great distances, sand so fine as this. 



Among the translucent particles of quartz, a considerable num- 

 ber of rounded, white, opake grains are perceived by the unassisted 

 eye — submitted to chemical tests these proved to be calcareous, 

 and under the microscope were seen to be polythalamia. They 

 were generally broken, worn and rounded. Six or seven spe- 

 cies only could be found, and of these Dr. Bacon could ascertain 

 only two, Textilaria glohilosa, and Rotalia globulosa ; another 

 species, is probably Textilaria aciculata. The cells of these 

 polythalamia are sometimes filled with calcareous matter, though 

 most frequently empty. Dr. Bacon did not consider himself au- 

 thorized to draw any inferences concerning the geological age of 

 the sands of Sahara, from the presence of those polythalamia, as 

 the species detected are found both in the cretaceous formations, 

 and in every age of the tertiary, and are believed to exist in a 

 living state in our modern seas. 



Dr. Bacon had also examined sand from the desert of Arabia, 

 described by Dr. Hitchcock, in a memoir on the Geology of 

 Western Asia, in the Transactions of American Geologists and 

 Naturalists. It^ general appearance is similar to that of the 

 specimens from the Sahara desert, except that the eye can de- 

 tect no calcareous particles in it, the quartzose grains are, how- 

 ever, larger, average .06 of an inch. The specimen examined 

 was entirely free from Polythalamia. 



