Memoirs of the Geological Survey of India . he harbour; ^nd, the plain which stretches northwardto the foot of the hills, a distance of about five and thirty miles; andthirdly, the hill ranges to the north, which are composed of stratified rocksand run in an east and west direction. It wag to the second of these, the country between the sea and the mountains near Aden^ that theGovernment suggested that attention should be directed, and -it was tothis plain that I devoted most time, but it will be well to indude thepeninsula and the hills also in the enquiry. Before describing the geological st
Memoirs of the Geological Survey of India . he harbour; ^nd, the plain which stretches northwardto the foot of the hills, a distance of about five and thirty miles; andthirdly, the hill ranges to the north, which are composed of stratified rocksand run in an east and west direction. It wag to the second of these, the country between the sea and the mountains near Aden^ that theGovernment suggested that attention should be directed, and -it was tothis plain that I devoted most time, but it will be well to indude thepeninsula and the hills also in the enquiry. Before describing the geological structure of Aden and _ the country to the north, it may be convenient toGeiieral condition!? re- ^ quired for Artesian wells, allude for the sake of comparison, to the generalconditions of stratigraphy under which artesian borings can be madewith success. The circumstances most favorable to such an under-taking are, when a thick bed of material (b) highly porous and permeableto water, such as sand or gravel, is intercalated between two beds. Fig-, i, Sketch section slicTFing-genefal couditiouii Iavoiable for altesian ( ) ADEN PENINSULA. 6 (a, c) of stiff clay or other impermeable material. These beds shouldform a more or less complete basin or synclinal, and it is necessary thatsome portion of the surface of the ground occupied by the bed c shouldbe at a lower level than the lowest point of the outcrop of h. Underthese circumstances, a certain proportion of the rain which falls on theoutcrop of h, and of the water which flows across it from and beyond a,will soak into the porous material and permeate down to the lowest level impermeable bed a prevents the water sinking lower, or keeps it up,while the bed c keeps it down. If then a boring be sunk at e down tothe permeable bed, the hydrostatic pressure of the column d h will forcethe water to rise in the borehole and overflow the surface of the statical rise of the water will be equal to e f, f being
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