4. (a) Alveolina zone 30 — 50 feet thick 

 (b) Shark's tooth shales 6 — 8 feet thick 



5. Chalky zone 110 — 220 feet thick 



6. Central brown limestone nowhere completely exposed 



The relation of these strata to one another and to the inner and outer scarps is 

 shown diagrammatically in figure 3. 



Two explanations have been advanced to account for this remarkable structure of 

 saucer and scarps. Pilgrim, who made the first sketch of the geology of the island (see 

 Mem. Geol. Survey India, XXXIV, pt. iv, 1908) thought these features were formed soon 

 after the island was first raised, by the ordinary processes of sub -aerial denudation 

 operating on the various strata described above at a time when rainfall was much greatei 

 than it is now. Later the island first sank and then emerged again so that the saucer 



Figure 2. 

 Diagrammatic sections across Bahrain Island from north to south and from west to east passing 

 through the Jebel Dukhan. The crosses indicate the Rim Rock or scarp of the central saucer. 



Vertical scale greatly exaggerated. 



Figure 3. 

 Ideal diagrammatic section, not to scale, across Bahrain Island along the line Buri, Jebel 

 Dukhan, Al Hisi to illustrate the geological structure of the island. The peripheral post- 

 Eocene deposits are stippled; the numbers are those of the six main strata, i.e. the white 

 limestone, the orange marl, the brown crystalline limestone, the Alveolina and shark's tooth 

 beds, the chalky zone, and the central brown limestone. AA is the outer scarp of which only 

 traces remain; BB is the Rim Rock or scarp of the saucer; and C is the summit of the Jebel. 



48 



