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Fig. 117. Top. Gulch irrigation of olive trees by the Matmata tribe of Berbers in Central Tunis. Rock dams have been built across the narrow valleys in limestone plateaus. Silt collects behind them. Every rain wets it all. The finest olive trees I have ever seen. Rainfall of 7 inches per year.—Fig. 118. Center. Bank between two rows of Tunisian olive trees to prevent run-off of surface water. Rainfall 7 to 8 inches. Trees 70 feet apart. Increase of trees gives no increase of fruit—Fig. 119. Bottom. Dr. J. H. Meyer, of Pennsylvania, at left, standing in a typical pit dug near an apple tree in his orchard to catch surplus water.
