202 JOSEPH THOMSON ON THE GEOLOGY OF [May 1899^ 



seem to have been much folded and now dip at a variety of high 

 angles. With each mile that the traveller penetrates into the heart 

 of the range the scenery develops in savage picturesqueness, owing to 

 the appearance of sandstones, which break the glen into more varied 

 irregularities. Gradually the sandstones increase in thickness, until 

 at last the shales practically disappear, and the glen narrows into 

 a forbidding gorge, dangerous and painful to traverse, alike for 

 man and beast. 



At length, after many hours of toilsome travelling, one reache& 

 the neighbourhood of Zarktan, there confronting the central mass 

 of the Atlas, opened up to view by the division of the Gadat 

 gorge into a series of radiating glens which spread themselves over 

 the face of the mountains. This sudden opening-up is evidently 

 due to a great fault running parallel with the mountains, by which 

 displacement of the rocks has taken place on a vast scale. On the 

 north side of this fault, and dipping in that direction, the above- 

 mentioned sandstones recur, while on the south lies a great series of 

 much-weathered red and purple shales dipping in a direction opposite 

 to that of the sandstones. In these have been excavated the glens 

 and valleys around Zarktan. 



Across these shales the traveller threads his way, finding it im- 

 possible to trace the sequence of the beds, until, reaching the base 

 of the central and highest mountain-range, he is confronted by 

 another great fault and another abrupt change in the character of 

 the rocks. Before reaching the last mountain- barrier the red shales 

 had passed into grey, and now at the fault are seen black shales 

 with a capping of quartzites. In remarking that the hollow, of 

 which Zarktan is the centre, is marked off north and south by 

 faults, I am irresistibly impelled to the conclusion that primarily the 

 hollow is due to a subsidence, and only secondarily to denudation. 



From this point onward, the Asif Adrar-n-Iri has carved a great 

 gorge or narrow glen, not merely deep into the heart of the main 

 chain, but practically right through it, wherefore the final pass by 

 which the traveller crosses lies at a comparatively low level on the 

 southern side. 



Near the end of the course of the Asif Adrar-n-Iri, at a locality 

 known as Titula, the glen opens out into a pear-shaped expansion, 

 and presents one of the most desolate views imaginable, in its 

 absence of vegetation and its gloomy colouring, due to the black 

 and grey shales and rusty quartzites. 



It is at Titula that one may first note undoubted evidence of the 

 former action of glaciers in the Atlas. On all sides masses of 

 debris, including huge angular blocks of quartzite, etc., meet the 

 eye in such conditions and positions as admit of no other explanation 

 of their existence than that of ice- action. 



At Titula also commences the final ascent of the Tizi-n-Teluet, 

 by which the southern slopes are gained, and the drainage-basin of 

 the Wad Draa is entered. The final barrier between the Wad Gadat 

 and the valley of Telnet consists of the narrowest of mountain- 



