196 JOSEPH THOMSON ON" THE GEOLOGY OF [May 1899,- 



holes as something beyond their knowledge. The cavity near 

 Mr. Hunot's farm is a circular pit, 30 or 40 feet in diameter and 

 from 20 to 30 feet deep. On one side rises a pillar of pure white 

 calcium carbonate, strangely unlike the reddish-yellow shell-rock 

 which surrounds it. This white carbonate spreads out, mushroom- 

 like, at the top, and forms a considerable deposit on the surface. 



The only theory by which I can account for this curious pit, 

 with its white pillar, is that at one time a hot spring, charged 

 with carbonate of lime, rose to the surface where now stands the 

 pillar, depositing its calcareous burden at its mouth. In time, the 

 pipe by which it found its way to the open air became choked by 

 encrustations of carbonate of lime, and then an explosion of steam 

 drilled a broader passage through the superincumbent rock, forming 

 the quarry-like cavity that now exists. The formation of the other 

 cavities may be reasonably ascribed to a similar cause. 



IV. The Plateau. 



On quitting the lowlands for the plateau the traveller practically^ 

 leaves behind the Tertiary area, and enters upon a region charac- 

 terized by metamorphic rocks and Cretaceous grey and cream- 

 coloured limestones, red sandstones, and red and purple clays. I 

 am aware that Maw suggests the possibility that some of the strata 

 in the plain of Morocco and on the flanks of the Atlas may yet 

 prove to be of Miocene age, but this supposition appears to me to 

 he founded on insufficient evidence. Certainly, as yet, no palaeon- 

 tological data have been brought forward to support this idea, and 

 the lithological evidence is of the most slender nature. In my 

 own opinion, the whole of the plateau of Southern Morocco has 

 been above water since the earliest Tertiary times. 



The metamorphic rocks cover the whole of Rahamna and 

 most of the great plain of Morocco, formed by the various tribu- 

 taries of the Tensift descending from the Atlas. In Eahamna and 

 Srarna they attain a considerable development, forming a very 

 broken and irregular series of mountains, termed Jebelet or Little 

 Mountain — the Atlas being the Jebel or Mountain proper. 



The Jebelet consists of much smashed and contorted clay-slates, 

 mostly lying vertical, with a general north-north-east and south- 

 south-westerly strike. These rocks, owing to their friability and 

 to the ease with which they split up, owing also to the prevalence 

 of more compact bosses and veins, form an extremely chaotic com- 

 plex of barren peaks and jagged ridges, having a general trend 

 parallel with the strike. Possibly these metamorjDhic mountains 

 represent the western, outcropping ends of a great syncline of 

 clay-slates, the plain of Morocco occupying the hollow ; while the 

 opposite or eastern end forms the core of the' central axis of the 

 Atlas itself. The southern limit of outcrop is not yet ascertained, 

 but the series is seen close to Amsmiz, and thence it probably 

 runs north-westward or north-north-westward to the Wad Tensift. 



