at some time every day, a wind of at least 10 miles an hour, and on two or three occa- 

 sions the daily minimum was 20 miles per hour, but some calm periods were recorded on 

 most days. The absolute maximum rate during this period was just over 50 miles per 

 hour. 



Bahrain Island 

 1. Structure and physiography. 



Geologically Bahrain Island is a simple shallow elongated anticlinal dome of Eo- 

 cene rocks, which dip down- flank from 1 to 5 degrees, and which are covered peripher- 

 ally and unconformably by more recent deposits. On the north this peripheral extension 

 of the island is considerable and mainly of rocks of Miocene age with a maximum thick- 

 ness of about 150 feet but these are partially or entirely covered with even younger 

 superficial deposits. The Miocene — Eocene boundary runs through the north part of 

 Sitra, across Nabbi Salih to Adari and Barbar. In this part also the island appears to 

 be growing by the gradual elevation of fringing coral reefs. In the south the peripheral 

 extension is even greater and again consists largely of Miocene rocks, with a thickness 

 of some 90 feet, but these are completely covered with younger deposits. Here there 

 are no coral reefs and the island tapers abruptly at its south end to a sharp point of 

 small sand-dunes. On the west the peripheral belt, though similar, is much narrower 

 and there are no reefs. Here again there is evidence of recent uplift in the presence of 

 a raised beach. On the east the peripheral belt is lacking for a distance of some miles. 

 The highest existing point of the Eocene dome, the summit of the massif known as the 

 Jebel Dukhan, is about 450 feet above sea level. 



Except for the peripheral deposits there are no major faults or unconformities in 

 the island and the simplicity of its anticlinal structure is complicated in only one im- 

 portant respect. This complication is that the whole central part of the island, com- 

 prising an area about 12 miles by 4, is a great shallow saucer with a slightly convex 

 floor, from which rises the Jebel Dukhan, and surrounded by a scarp cliff, called by the 

 petroleum geologists the Rim Rock, which averages about 50 feet in height. The Jebel 

 Dukhan is slightly west and north of the exact middle of the saucer and the floor is 

 rather lower in the south and parts of the west (where it is not more than 50 feet above 

 the level of the sea) than in the east and north (where it is about 100 feet above the 

 sea). Figure 2 shows diagrammatic sections of the island along the two main axes with 

 the vertical scale very greatly exaggerated. 



In the northern part of the island there are in two places, near Buri and A\ Hisi, 

 lengths of other scarp cliffs very like those of the Rim Rock, and these are apparently 

 all that now remains of an outer scarp cliff. 



The main strata of the Eocene rocks on the island are seven, namely, from above 

 downwards :- 



1. White limestone - 0-150 feet thick 



2. Orange Marl 30 - 50 feet thick 



3. Brown crystalline limestone — Nummulitic limestone 100 — 150 feet thick 



47 



