GEOLOGY OF MAEOCCO AND GEEAT ATLAS. 45.5 



valley bottoms; and the only satisfactory explanation of its 

 origin I can suggest is, that it results from the intense heat of 

 the sun rapidly drawing up -water charged with soluble car- 

 bonate of lime from the calcareous strata, and drying it layer 

 by layer on the surface, till an accumulation several feet thick 

 has been produced. The rapid alternations of heavy rains and 

 scorching heat which take place in the Marocco plain are con- 

 ditions favourable to this phenomenon, which is unknown in 

 northern temperate climates. 



Fia. 3. 



A familiar illustration of the same kind of actioti is seen in 

 what brickmakers term ' limewash.* A brick fornied of marl 

 containing soluble carbonate of lime, if rapidly dried or placed 

 in the clamp in a wet state, will have on its upper surface, 

 after burning, an tmsightly white scum or crust, by the accretion 

 of soluble matter driven upwards and outwards by the quick 

 evaporation. Before we left Mogador on our journey inland, 



