Even before such a range attains its maximum height a very considerable amount of erosion has already taken place. When the first fold appears above sea level, erosion begins its work and continues with increasing vigor as the mountain masses get higher and higher. Thus we have warfare between two great natural processes--the building up and the tearing down. After a time the building-up process wanes and then ceases, while the tearing-down process (erosion) continues either until the whole range has been completely worn down or until some rejuvenating force causes a renewed uplift. Here is an example of one of the remarkable procedures of nature. After millions of years of work causing the deposition of thousands of feet of strata, piled layer upon layer on the sea floor, a force of lateral pressure is brought to bear and a mountain range is literally born out of the sea. No sooner is the range well formed than the destructive processes (erosion) unceasingly set to work to destroy this marvelous work. But the sediments derived from the wear of the range are carried into the nearest ocean again to accumulate and, perchance, after long ages, to be raised into another range; and so the process may be often repeated. From this we learn that the mountain ranges of the earth are by no means all of the same age. The original Adirondacks were formed long before the Taconics, which originated millions of years before the Appalachians, these latter having been folded up long before the Sierras. The Rockies, followed by the Coast Ranges, are each younger than the Sierras as regards their original folding and uplift. Among foreign countries special mention should be made of the British Isles, where Ordovician strata thousands of feet thick were, late in the period, notably folded and upraised, the crustal disturbance having been accompanied by great intrusions of molten rocks and vast outpourings of lavas, so that this region ranks among the greatest of the ancient volcanic areas of Europe. [Illustration: Fig. 37.--Map showing the general relations of land and water in North America during middle and late middle Devonian time fully 15,000,000 years ago. (After Willis, courtesy of the Journal of Geology.)]