and of the East African Mainland. 



37 



but the coral rock is confined more to the edges of the island than 

 in the latter case, appearing on the surface only as a narrow belt 

 along the east coast, and forming the outermost islets of the west 



Immediately within this barrier the surface is of coherent sand, 

 beneath which is an extremely soft sandstone 1 lying above the coral 



1 This contains rounded pebbles of quartz smaller and less coloured than those 

 occurring in Zanzibar. 



