From Cabo Cruz the plain extends along the northern base of the Sierra Maestra to the head of the Cauto Valley. Its elevation near Manzanillo is about 200 feet, whence it increases to 640 feet at El Cristo. In the central section of Oriente, the Cauto River and its tributaries have cut channels into this plain from 50 to 200 feet in depth. In the lower part of the valley these channels are sometimes several miles across and are occupied by alluvial flats or river bottoms. They decrease in width toward the east and in the upper part of the valley become narrow gorges. A large part of this plain of Cuba, especially in the central provinces, is underlaid by porous limestone, through which the surface waters have found underground passages. This accounts for the fact that large areas are occasionally devoid of flowing surface streams. The rain water sinks into the ground as soon as it falls, and after flowing long distances under ground, emerges into bold springs, such as those of the Almendares that burst out of the river bank some eight miles south of the City of Havana. Engineers of the rope and cordage plant, just north of the City of Matanzas, while boring for water, found unexpectedly a swift, running river, only ten feet below the surface, that has given them an inexhaustible supply of excellent water. Most of the plains of Cuba above indicated have been formed by the erosion of its surface, and are covered with residual soil derived from the underlying limestones. Where they consist of red or black clays they are, as a rule, exceedingly fertile. Certain portions of the plains, especially those bordering on the southern side of the mountains of Pinar del Rio, are covered with a layer of sand and gravel, washed down from the adjoining highlands, and are, as a rule, inferior in fertility to soils derived from the erosion of limestone. Similar superficial deposits are met in the vicinity of Cienfuegos, and in other sections of the island, where the plain forms a piedmont adjacent to highlands composed of silicious rocks. The most striking and perhaps the most important fact in regard to the climate of Cuba is its freedom from those extremes of temperature which are considered prejudicial to health in any country. The difference between the mean annual temperature of winter and that of summer is only twelve degrees, or from 76 degrees to 88 degrees. Even between the coldest days of winter, when the mercury once went as low as 58 degrees, and the extreme limit of summer, registered as 92 degrees, we have a difference of only 34 degrees; and the extremes of summer are seldom noticed, since the fresh northeast trade winds coming from the Atlantic sweep across the island, carrying away with them the heated atmosphere of the interior.