112 ORIftIN OF FLAT-TOPPED HILLS. ch. t. 



above the well. The scarped face exhibited a section of 

 the yellowish-white limestone that appears to underlie 

 nearly the whole of the low country between the coast 

 and the base of the Atlas. No fossils were found ; and in 

 the present state of our knowledge, or rather ignorance, 

 of the whole region, it seems impossible to fix its position 

 ill the geological series. The level summit is capped by 

 a thin layer of coarse chalcedony, in which we recognised 

 tlie origin of the siliceous fragments scattered over the 

 plain below. This layer would offer resistance to superfi- 

 cial denudation, and account for the tabular forms of the 

 hills, but where these were attacked from below by marine 

 or river action the covering would necessarily be broken 

 up and the fragments scattered over the plain below. 

 With reference to the opinion expressed by Maw in his 

 paper in the Proceedings of the Geological Society, and 

 in the Appendix to this volume, as to the origin of the 

 tufaceous coating of the plain between Ain Oumast and 

 Marocco, the only difiiculty that presents itself arises from 

 the presence of these siliceous fragments on the surface 

 along with the disintegrated tufa. If, as he and other 

 geologists believe, such a superficial coating is due to 

 evaporation from the underlying mass of water charged 

 with carbonate of lime, it seems hard to account for the 

 diffusion of the chal6edony fragments, unless we suppose a 

 submergence of the plain subsequent to the formation of 

 the tufa layer, and a renewed supply of such fragments 

 by further erosion of the hills that formed the sea or river 

 coast line. To confirm this conjecture, we may note the 

 fact that the fragments of chalcedony became progress- 

 ively rarer as we advanced from the lower portion of the 

 plain over which we this day travelled to the upper level 

 surrounding the city of Marocco. 



The summit of the hill was found to be 1,648 feet 

 (502*4 m.) above the sea and 303 feet above the well at its 

 base. It was barren, yet supplied a few additional plants 

 to our collection. Frankenia revoluta was abundant, as 



