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Show RICHTHOFEN'S ORDER OF SUCCESSION OF ERUPTIONS. 63 living observer has studied this problem more carefully nor included in his observations and generalizations a wider field. His extensive knowledge, his great acumen, and his ability to generalize brilliantly, though cautiously, entitle his conclusions to the most earnest consideration. As the result of his study of volcanic phenomena in many portions of the world, he believes that the various kinds of eruptive rocks reveal a certain order of succession in their relative ages of eruption throughout Tertiary time. Arranging these rocks according to their physical properties and intimate constitution into five groups, or orders, he finds that they have been erupted in the following sequence • 1. Propylite. 2. Andesite. 3. Trachyte. 4. Rhyolite. 5. Basalt. It will seldom happen that more than two or three of these kinds of rock will be found in direct superposition, the series in any given locality being always incomplete, and in very many cases a single kind will alone be found. But wherever two or more are found superposed, the one having the prior enumeration in the foregoing list will be the older. The only exceptions would be where each order of rocks is represented by numerous individual outbreaks, when the later extravasations of the older order may occasionally be seen to intercalate with the older extravasations of the later order. These considerations apply to what are termed "massive eruptions," where deluges of lava have broken forth from fissures and overwhelmed the adjoining regions with coulees far exceeding the ordinary emanations of common volcanoes. They also apply to the history of those grander vents which have maintained an activity lasting through a considerable proportion of Tertiary time. But the smaller vents as a rule are of very brief geological duration, and seldom disgorge more than one kind of lava. In support of his generalizations he adduces his own extended observations in Hungary, Germany, and the Sierra Nevada, and those of many colabor-ers in Armenia, Mexico, Central and South America. Those geologists who have made a special study of the volcanic rocks |