NOMES XI. AND Xlt. 249 



tant regions ; those empires of sand present to 

 human observation no symptom of utility, but, 

 on the contrary, daily encroach on the fertile 

 vales in their vicinity. 



Sandstone rock and sand, from the desarts of 

 Africa. 



The same, from Arabia. The sand is red and 

 coarse, and the decomposition would appear to 

 proceed from iron ; so that a metal of the great- 

 est utility may, in the field of battle, or in the 

 dreary desart, become the most pernicious to the 

 human race. 



Sandstone and sand, from the desart of 

 Shamo. 



NOME XI. D. CLAY-SLATE. 



This is a common occurrence. Aluminous 

 slate is particularly subject to decomposition. 



NOME XII. D. SAUSSURITE. 



This magnesian basaltin, one of the pierres 

 de come of Saussure, is not only liable to a su- 

 perficial decomposition, forming a white crust; 

 but, as it sometimes contains asbestos and ami- 



