from the rainfall of Central Arabia and even from regions further north-west, but some 

 of the shallower wells derive their supplies from local rainfall catchment. Unfortunatel] 

 all this water (except for one or two shallow wells) is highly brackish, that from the 

 north of the island commonly containing between two and three thousand parts per mil- 

 lion of dissolved salts, and that from the south as much as four thousand parts. It is 

 owing to this circumstance that despite the amount of water available for irrigation the 

 only crops which flourish on a considerable scale are dates and lucerne. 



2. Natural Areas. 



In the simplest terms Bahrain and its three most closely associated islands con- 

 sist of two natural regions only, those of the peripheral post- Eocene deposits, roughly 

 outlined by the 50 ft. contour line, and the central Eocene dome, but each of these is 

 further divisible. 



With regard to the first various circumstances, of which the distribution and acces- 

 sibility of water is probably the most important, combine to make the northern part of 

 the area, and especially that north of the Miocene boundary, of much greater potentia- 

 lity as a human habitat than the rest of the island. In consequence nearly the whole 

 population is here and human exploitation of every sort diminishes very rapidly towards 

 the south. In latter years it is true that the building of the oil company's desert town 

 of Awali at the north end of the saucer has in some measure distorted this picture in 

 fact though not in theory, since its presence there has been made possible only by over- 

 coming the natural limitations of the site by purely artificial means. 



The direction of the prevailing wind from the north-west adds a north-west to 

 south-east component to this southward gradient, partly by its effect on the distribu- 

 tion of rainfall, and partly by the accumulation of blown sand in its direction. As a re- 

 sult there is, in addition to a diminishing human gradient from north to south a dimini- 

 shing vegetational (fertility) gradient from north-west to south- east, an effect which is 

 particularly noticeable within the saucer. 



The PERIPHERAL (post- Eocene) ZONE which, it will be recalled, is absent alon^ 

 the central part of the east coast, can be divided into a northern cultivated area and a 

 western and southern almost uninhabited part. The boundary between the two in the 

 north-west of the island is not however clearly marked since there are areas of more or 

 less natural desert almost to the north coast, while there are scattered date gardens far 

 to the south. The cultivated area has several aspects ; the western and southern area 

 is more monotonous, its chief feature being an extensive shallow pan running north- 

 west from Mattala. 



The CENTRAL (Eocene) DOME divides into several constituent parts in accordanc 

 with the physiography illustrated in figures 2 and 3, namely the FLANKS; the SAUCER 

 the CENTRAL PLATEAU and the JEBEL DUKHAN. The first of these is complex in 

 that it is locally double where the outer scarps occur and the white limestone is expose( 

 but on the main continuous flanks outside the saucer it is the brown crystalline lime- 

 stone that provides the surface, except for some local patches of orange marl. The sur- 

 face of the saucer is of beds of the chalky zone, modified to a varying degree by the 

 products of erosion of the central plateau and the Jebel. The central plateau is formed 



50 



