Vol. 55.] GEOLOGY OF SOUTHERlSr MOROCCO AND THE ATLAS. 201 



to the other, where it escapes in a waterfall, while it also forms an 

 invaluable means of communication for the inhabitants of the 

 opposing slopes. 



Above Iminiiiri, as this singular natural bridge is called, the 

 limestones are seen to form a comparatively shallow valley or 

 mountain-step, broken somewhat by a series of ridges which are 

 seen to coincide with synclines, off which the red shales and 

 sandstones that occupy the hollows have been denuded. 



Beyond this middle section of the course of the Wad Demnat, the 

 Irghalnsor mountain towers abruptly to a height of 5500 feet, its 

 steep face coinciding with the northerly dip of the limestones. 

 Between Irghalnsor and the main mass of the mountains lies the 

 deep glen of the upper course of the Wad Demnat, carved out of 

 the friable red shales and sandstones. 



Standing on this coign of vantage, it is not difficult for the observer 

 to perceive that the massive, almost unbroken wall, which rises so 

 majestically before him to a height of 10,000 feet, forming the 

 central crest of the Atlas, is composed of sedimentary rocks similar 

 to those upon which he stands, a conclusion verified farther west 

 when the mountains are crossed from north to south. 



An excursion farther west, to Tasemset, and the excavations of 

 the Wad Tessaout reveal a similar succession : the outer flanking 

 limestones, in vertical or highly contorted beds, folded into V's 

 and W's ; an enormous basalt-dyke parallel with the axis of the 

 mountains ; and a middle zone of limestones and shales, rising in 

 broken scarped folds in ever-increasing magnitude, till they sweep 

 skyward in one grand culminating mass. 



It is along this section that the Upper Cretaceous rocks attain 

 their greatest development, masking indeed, so far as the secondary 

 ranges are concerned, the lower shales and sandstones. 



(2) The Wad Gadat. (Fig. 2, p. 200.) 



We may pass now to the section which, following the Wad Gadat, 

 crosses to the valley of Teluet, on the southern flank of the range. 

 Starting from Tezert, about 4 miles east of the Gadat, the traveller's 

 route takes him over some low hills chiefly made up of friable basalt, 

 which possibly may be a continuation of the Demnat dyke, occupying 

 as it does the same position at the base of the mountains. Prom 

 the basalt-hills one descends into the small gien of the Wad Lar, 

 where the limestones occur, much altered by the intruded igneous 

 rock. At this point the traveller leaves the Upper Series and enters 

 upon the area of the Lower Beds, which here consist almost exclu- 

 sively of friable red and purple shales with numerous brine-springs. 

 The same formation continues as far as Enzel on the Wad Gadat, 

 where the mountains rise abruptly to a height of over 8000 feet, and 

 largely composes these mountains. 



In following the course of the Gadat a great narrow glen is 

 entered, with clay-cliffs rising almost perpendicularly several 

 thousand feet above the river. Owing to the decomposed condition 

 of these clays, it is almost impossible to trace their bedding, but they 



