The rim dividing the Mississippi basin from that of the Great Lakes is, as we have seen, low and narrow; in some places, especially in wet seasons, the watershed is indistinguishable. The waters know not which way to go. This fact furnishes the explanation of the ease with which the French explorers penetrated the valley from the north. A high mountain range kept the English colonists out of it from the east. The Spanish found no physical barriers at the south (except the water, which gave the Frenchmen help), but, as we have seen, on the other hand, they found no adequate inducement. The isotherm which touches the southern limits of France passes midway between the source and mouth of the river. In the northern half, it has the mean annual temperature of France, England, and Germany; in the southern half, of the Mediterranean coasts. From the gulf into which it empties, a river (that is, an ocean river, or current) runs through the ocean to the western coasts of Europe; another runs out along the northeastern coast of South America, and, still another is in waiting at the western terminus of the Panama Canal to assist the ships across the Pacific. A fair regularity and reliability of rainfall have made the rich soil of the valley tillable and productive without irrigation, except in the far western stretches; and these blessings are likely to continue, as one authority puts it, "so long as the earth continues to revolve toward the east and the present relationship of ocean and continent continues." Including Texas and Alabama (which lie between the same ranges of mountains with this valley, though their rivers run into the gulf and not into the Mississippi), this valley has perhaps one hundred and forty thousand miles of railway, or about sixty per cent of the total mileage of the country, or twenty-five per cent of the mileage of the entire globe. "In richness of soil, variety of climate, number and value of products, facilities for communication and general conditions of wealth and prosperity, the Mississippi Valley surpasses anything known to the Old World as well as the New." It produces the bulk of the world's cotton and oil; of corn it raises much more than all the rest of the world combined, and of each of the following (produced mainly in this same valley) the United States leads in quantity all the nations of the earth: wheat, cattle, hogs, oats, hay, lumber, coal, iron and steel, and other mineral products.