206 JOSEPH THOMSON ON THE GEOLOGY OF [May 1 899, 



(4) The Wad Urika. (Fig. 3, p. 204.) 



We now come to the Wad Urika, the glen of which, despite 

 Sir Joseph Hooker's and my own elSPorts, remains only partially 

 explored. 



As in the case of the entrances to the glens of the Wads Demnat 

 and Gadat, that of Urika is marked by a series of limestones thrown 

 into a vertical position by an intrusion of basalt. The limestone- 

 beds have but a slight development here, and are replaced on the 

 opposite side of the basalt-dyke by purple shales and sandstones, 

 with beds of what appears to be volcanic breccia dipping at a low 

 angle northward. 



These shales and sandstones form a series of comparatively low 

 hills, till some 3 miles from the plain a fault occurs, and a series 

 of fine-grained red sandstones rise abruptly to a height of several 

 thousand feet. Close to the fault is an outcrop of crumpled 

 dark shales, which may belong to the same series as those observed 

 on the Wad Gadat. Near the same point, the eastern side of the 

 glen is marked by a narrow platform of debris, 200 or 300 feet 

 thick, and rising to that extent in a bold precipice above the river. 

 The size and shape of the contained boulders and the general 

 characteristics of the deposit point to a glacial origin, but on this 

 point I should not care to express any very decided opinion. 



As hitherto no traveller has been able to penetrate farther than 

 the fault, one can but theorize as to the composition of the 

 mountains beyond; but, to judge from the river-debris, it seems 

 evident that the central mass is composed of metamorphic and 

 igneous rocks. 



(5) The Wad Reraya. (Fig. 4, p. 204.) 



We now come to the important glen of the Wad Eeraya, which 

 penetrates to the very heart of the Atlas. It has been examined, 

 not only by myself, but by Sir Joseph Hooker, the latter having 

 followed the Ait Mesan or western division of its upper course, 

 while I explored the Iminnen or eastern branch. 



At Taghnowt the glen is comparatively open, displaying in its 

 sides outcrops of white limestone and red shale which form a small 

 anticline. A mile farther up, the glen suddenly narrows, the con- 

 traction being due to an outcrop of metamorphic rocks, whose 

 junction with the shales is marked by a fault. For a couple of 

 miles the river winds in a narrow gorge through these rocks, till 

 once more the Cretaceous Series reappears, dipping at a low angle 

 southward. So far, a southerly course has been followed, but now 

 the traveller turns suddenly westward, to find that he is following 

 the outcrop of a basalt-dyke, from which an open valley has been 

 excavated. On either hand, heaps of travelled debris are seen to 

 cluster on the mountain -sides, and owe their position and origin 

 probably to ice-action. 



