Most of Africa's population is rural, but, except for cash crops, such as cacao and peanuts, agricultural production is low by world standards; Africa produces three quarters of the world's cocoa beans and about one third of its peanuts. Rare and precious minerals (including much of the world's diamonds) are abundant in the continent's ancient crystalline rocks, which are found mostly to the south and east of a line from the Gulf of Guinea to the Sinai Peninsula; extensive oil, gas, and phosphate deposits occur in sedimentary rocks to the north and west of this general line. Manufacturing is concentrated in the Republic of South Africa and in N Africa (especially Egypt and Algeria). Despite Africa's enormous potential for hydroelectric power production, only a small percentage of it has been developed. Africa's fairly regular coastline affords few natural harbors, and the shallowness of coastal waters makes it difficult for large ships to approach the shore; deepwater ports, protected by breakwaters, have been built offshore to facilitate commerce and trade. Major fishing grounds are found over the wider sections of the continental shelf as off NW, SW, and S Africa and NW Madagascar.