C. Exposed limestone is found on the flanks where intermittent surface drainage has 

 scoured away some or all of the overlying sand and gravel, namely in the numerous small 

 wadis which are thickly distributed here, but occurs also in parts of the central plateau 

 where wind erosion has perhaps played a larger part. According to the steepness and 

 nature of the substrate occasional heavy rain may pass through these wadis almost in a 

 torrent, but elsewhere the flow may for various reasons be slower so that there may even 

 temporarily be pools of standing water. It is in these flatter wadis that the richest and 

 most luxuriant vegetation of the island is to be seen, in the form of thickets several feet 

 high composed of such shrubby plants as Zizyphus and Atriplex and various large herbs 

 and grasses. These wadis also show the largest collections of species, one of them 

 for example, south-east of the village of Ali, containing upwards of 90 species, or 

 about half the whole desert flora. 



On the central feature of the island, the Jebel Dukhan, all three main types of des- 

 ert occur; sandy desert on parts of the north slopes, where wind has deposited material, 

 stony desert on the summits and flanks where the rock is not fully denuded and exposed, 

 and exposed limestone on the rest of the summits and on the flanks where slope and 

 other factors preclude any more superficial deposit. The Jebel presents two habitat 

 features peculiar to itself and its minor homologues, namely the presence of slopes 

 steeper than found elsewhere and locally even precipitous, and in consequence the oc- 

 currence also locally of shaded niches such as do not occur on the open desert. It is 

 interesting to note however that while various species attain an unusual size in such 

 favoured spots there are apparently no species peculiar to them. About 75 species 

 occur on the Jebel. 



The parts of the saucer which do not bear typical sandy desert are for the most 

 part in something between this condition and stony desert but very notably on the west 

 side the white, marly beds of the chalky zone are bare, and here there are large patches 

 of vegetation of considerable local repute for horse and other grazing owing to the pre- 

 sence of 'sorrel' (apparently Emex spinosus) and certain other plants. 



The Island of Umm Nasan 



This has an area of about 9 square miles. It is used as a hunting preserve by the 

 Sheikh and is inhabited only by a few of his retainers. 



Apart from some very local and shallow limestone exposures on the shore the whole 

 island is a flat sandy plain from which emerge two low isolated rocky hills of which the 

 larger is about 70 feet high. Owing to the extreme flatness of the rest of the island 

 these hills have a prominence out of all proportion to their size and are visible from a 

 considerable distance. 



The sandy plain compares with the peripheral zone of Bahrain and is presumably 

 of similar age, and bears a well - developed community of about a dozen species all of 

 which occur also on the larger island. The hills each compare with the Jebel Dukhan 

 on a minute scale and the larger has 35 of the same species. 



The Island of Jedda 



This island, which is about a mile round, is used as a prison. It is a solid flat- 

 topped mass of limestone with practically no peripheral zone and is almost everywhere 



53 



