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pernicious type. It had been noted for some time previously that com- 

 panies going for target practice to the marine rifle range at Maquinaya, 

 some 5 kilometers north of Olongapo on the east shore of Subig Bay, 

 invariably returned to Olongapo with a large percentage of their person- 

 nel infected with malaria, in one instance as many as 85 per cent having 

 the fever. 



It was not possible, owing to pressure of other work in Manila, for me 

 to reach Olongapo before the 1st of April, so that upon my arrival I 

 found the disease to have diminished markedly, although cases were b) r 

 no means of rare occurrence. 



GEOGRAPHY AND TOPOGRAPHY. 



The naval station of Olongapo is situated at the southeast end of the 

 small island of that name, lying in the eastern part of Subig Bay and 

 separated from the mainland of Luzon by the Kalaklan Biver. It is 

 really one of a series of deltas formed at the mouths of the Kalaklan, 

 Santa Bita and Biniktigan Bivers. The region roundabout is hilly, 

 often to the coast, the main axes of the hills running at an angle with 

 the coast line and having broad valleys between them, especially on the 

 east shore of the bay. More or less sluggish streams flow through these 

 valleys and upon reaching the coast they naturally spread out, forming 

 areas of mangrove swamp in which a certain species of large crustacean 

 has built up mounds of earth which in time, owing to vegetation and the 

 further deposit of earth by the rivers, have united to form a more or less 

 level tract having an elevation of from 3 to 10 decimeters above mean tide. 

 Numerous channels are cut through these tracts of land at the time of 

 floods, and when the tides are high the waters of these channels are con- 

 tinuously mixed with that from the sea. At no time is the water in or 

 around these areas fresh, and where the tide recedes it leaves the land- 

 locked pools to become more salty by evaporation. The shores for several 

 kilometers around Olongapo in all directions are of this character, and this 

 condition is only relieved by the terminal talus slopes of the surrounding 

 hills. 



Engineering operations carried on by the Navy with reference to harbor 

 improvements at the station, have altered the course of the Biniktigan 

 Biver while causing large bodies of water to become more landlocked 

 than previously, with the result that now the area of stagnant or semi- 

 stagnant water in the vicinity of the station is about doubled. 



The same condition obtains at the Maquinaya rifle range, except that 

 the swamps at this place lie farther landward, whereas the camp itself 

 is located on a sandy beach, upon which fair sized trees of various species 

 grow. 



