The Canal Zone, ten miles wide and forty-five miles long, is composed of mountains of moderate height, marshy swamps, numerous small lakes, jungles, almost impenetrable in some places, and streams, the most important of the latter being the Chagres Eiver, celebrated for malignant malarial disease. The temperature ranges from 65° to 100°, March being the hottest month. The average annual rainfall varies strangely in different localities from 75 to 125 inches. The fog, clouds and hot sun follow each other in quick succession. The heavy rainfall insures permanent stagnant water where the larvae of the yellow fever and malarial mosquitos thrive in countless millions; the perpetual moisture, warmth and rich soil lead to extravagant growth of hundreds of varieties of tropical grasses, plants, flowers, vines and trees, furnishing favorable harbor for the insects; and there is an almost constant stream of decaying vegetable and animal matter pouring into lakes and marshes that are never drained. Decaying animal matter leads to the generation of innumerable flies, ever ready to convey disease, and the water supply is polluted, and pregnant with disease germs.