The pebbles, though black predominate, are of a most remarkable variety of kinds and colours. Bright green and red, yellow and clear white are abundant, and any square yard would yield a rich collection in Petrology. As the torrents open out into the level plain they lose themselves, continually taking to fresh channels, so that the _débris_ from series of hills quite distant from one another are mingled; in a given spot gravel from one valley is laid down this year, from another and totally distinct one another. One would expect gravel which had been carried by torrents a distance of many miles to be rounded down by friction into smooth boulders or pebbles, like those of our home streams. As a matter of fact it is nearly always angular, the rounded surfaces we should expect being rarely met with on the surface. The pebbles, as we now see them, have been re-formed from larger stones since their transport through the valleys and over the plain. Large stones, lying half buried in smaller material, shew the usual rounded surfaces of water-borne rock, but they are invariably split up by fissures, which may be half an inch broad, so that the stone is as it were built up of angular fragments fitted together after the style of a puzzle picture. During the hundreds of years they have lain there, apparently secure from all interference, they have been exposed to innumerable fierce heats and cold nights, which, causing successive minute expansions and contractions, have at last split the stones into small pieces. This is the origin of the irregularly shaped gravel; first indeed it was rounded by the grinding and pounding of the torrents of hundreds of successive winters, then it was split up again by the silent invisible stresses of heat and cold. _Plate XXXVI_ [Illustration: Fig. 79. The maritime plain and Irba Mountains Yemêna oasis in foreground and gravel-covered ridge across middle distance] _Plate XXXVII_ [Illustration] [Illustration: Figs. 80 and 81. Corals, in position of growth, on the summit of Jebel Têtâwib] The accumulation of this vast mass of gravel and sand in the manner described has taken a length of time compared to which a human life is but a moment. Even from the geological point of view it has been not inconsiderable. There is abundant evidence that the plain was formed, much as it is now, at a time when the coast-line was entirely different, and though there is no good evidence of the country’s having been other than desert throughout historical time, there probably was a greater rainfall when the formation of the plain was in full swing.