Page:Popular Science Monthly Volume 77.djvu/609

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N the course of a fourth journey among the islands of the Pacific Ocean, during the year 1909, the rare opportunity was presented of making an ascent of the remarkably active volcano formed about five years ago on the island of Savaii, the largest member of the Samoan group. In addition, during a short stay in the Hawaiian Islands, a visit was made to Kilauea, a volcano which in contrast with the former, has a long geological history, for records of its intermittent periods of activity cover more than a century. It is the purpose of the present article to give a short general description of these two volcanoes.

Under any circumstances such works of nature would arouse the interest of a student of natural phenomena; but in my own case the opportunity to study them was valuable for additional reasons. My investigations of the distribution and evolution of the land snails of Polynesia demanded a thorough exploration of volcanic islands of greater age, islands that for many centuries have been sculptured by the elements so as to display alternating ridges and valleys radiating from their high central peaks. Tahiti is perhaps the most beautiful example of such an island. One finds that the several islands of the Pacific groups are of various geological ages, and consequently exhibit different degrees of weathering. They thus constitute a series showing how ancient rugged islands like Tahiti and Moorea have been derived from newly formed volcanic mountains like those of the Hawaiian and other groups, which possess relatively even sides of lava fields unfurrowed by erosion. Furthermore the various islands scattered throughout the vast areas of the Pacific Ocean are interesting to the naturalist because of the evidences they give of great changes in the level of the ocean bed, and also on account of the role played by corals in the construction of many types of islands. With few exceptions the islands occur in groups or chains suggesting the conclusion that they are the peaks of a range of mountains formerly connected by lowlands but now separated as the result of a subsidence of the ocean's floor. Every one is familiar with the theory that a coral atoll, consisting of a living reef bearing a more or less extensive series of coral islets, is built upon such a volcanic peak, which, according to Darwin and Dana, has been withdrawn below the water's level and overgrown by coral as it slowly subsided. It may be, as Agassiz contends, that a coral