tLAIN KORTH OF ADEN, 21 



mode of formation of the plain from detrital matter brought down to 

 the sea by rivers (mostly during floods), there is little reason to expect 

 any transition lower down^-io plastic clay or other impermeable material. 

 It seems to me by no means improbable that the gravel extends down 

 to the solid rock beneath, which would perhaps be found at no very 

 great depth. Even if an impervious stratum should exist beneath the 

 gravel, it would be necessary that the lower porous stratum should 

 outcrop to the north, at a level sujEficiently above that of the surface of 

 the ground at the proposed well, to provide sufficent hydrodynamic 

 pressure to overcome the friction of the water through the gravel. The 

 very gentle inclination of the plain does not favour such an arrangement, 

 and even were it found to exist, — unless by thinning out of the water- 

 bearing stratum between two impervious ones or other means, the water 

 were prevented from flowing on direct through it to the sea, — no sufii- 

 cient pressure could be expected, to force it up the borehole or well. 



The sandhills to the north, consisting, as they undoubtedly do, of 

 loose sand for several hundred feet in depth, are of course quite out of 

 the question in respect to artesian borings. 



It would be useless to speculate on the solid rock beneath the plain 

 without accurately knowing what it is, whether volcanic, metamorphic or 

 stratified, or anything of its stratigraphical relations ; but from all I know 

 of the rocks of the country where they are visible, I see no reason 

 whatever to expect a successful result in sinking down to them beneath 

 the plain. 



The stratum of sand and gravel beneath the plain contains, as I 



have said before, an abundant supply of water 

 Common wells. 



stored up in it, although such cannot be made 



available by artesian wells. The river which leaves the hills to the west 



of Jibal Manif, has a bed at Zaida, as observed by Captain Mander of 



300 yards in breadth. Where I crosssed the western branch (Wadi el 



Kebir) below Lahej on the 27th November, the stream was about 



( 277 ) 



