Surface.
Section.
A familiar illustration of the same kind of action is seen in what brickmakers term ‘limewash.’ A brick formed 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 unsightly 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, we were told of great beds of shingle covering the plain, and fully anticipated some interesting drift phenomena; but these shingle-beds were found to be nothing more than the broken débris of the surface tufa, covering the plain for hundreds of square miles with stony fragments. Of marine drift there is not a vestige, the few isolated patches of waterworn stones and alluvial shingle being always connected with river valleys, excepting only the huge boulder deposits of the Atlas hereafter to be referred to.
About midway between Mogador and the city of Marocco, the monotony of the plain is broken by a curious group of flat-topped hills, which rise two or three hundred feet above its
Fig. 4.
‘Camel’s Back,’ flat-topped hills in the Plain of Marocco.
general surface. They present straight scarped sides, on which are exposed cream-coloured calcareous strata capped with a flat tabular layer of chalcedony, which seems, in arresting denudation, to have determined their peculiar and symmetrical form. In these we found no fossils; and I am doubtful whether they are an inland extension of the Miocene beds observed by Dr. Hooker at the ‘Jew’s Cliff,’ near Saffi, or are some members of the Cretaceous series, of which there are sections on the coast north of Saffi and on the flanks of the Atlas.
At this point the main boundaries of the plain come into full view,—on the north a rugged range of mountains trending east and west, which we estimated at from 2,000 to 3,000 feet in height; and on our right the great chain of the Atlas, rising 11,000 feet above us and between 12,000 and 13,000 feet above the sea, bounds the view to the south, framing-in the great plain, here some 50 miles broad, which is lost as a level horizon in the eastern distance.