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Show THE OBSERVED ORDER OF SUCCESSION OF ERTJPTIOX3. 67 properties of the rocks. This inquiry immediately presents itself the instant we settle upon the conviction that eruptions have an assignable order of occurrence, and the mind at once springs to the conclusion that there ought to be such an association. If there be an order of eruption, there must be a cause for it, and for that cause we look to the properties of the rocks themselves. But at first glance no such correlation appears. If we arrange them in a series expressing the great groups in the order of their chemical constitution, and place in juxtaposition an arrangement according to the order of eruption, we fail to find at first a clear correlation. Taking Richt-hofen's five orders, we have the following comparison: Arrangement by chemical constitution. 1. Rhyolite. 2. Trachyte. 3. Propylite. 4. Andesite. 5. Basalt. Arrangement by order of eruption. 1. Propylite. 2. Andesite. 3. Trachyte. 4. Rhyolite. 5. Basalt. With chemical constitution go the other properties, mineral constituents and specific gravity. No relation here presents itself to the order of eruption. Yet I think that upon closer inspection a systematic correlation may be made to appear by an examination of the sub-groups instead of the great groups, and the correlation of the sub-groups will reflect itself in the great groups. Taking the more important sub-groups, those which are most persistent in their characters, of most frequent occurrence, and of the largest volume, the following succession of eruptions presents itself in the High Plateaus :* 1. Hornblendic propylite. 2. Hornblendic andesite. 3. Hornblendic and augitic trachytes (less acid trachytes). 4. Augitic andesite (Richthofen). 5. Sanidin trachyte (more acid trachytes). 6. Liparite. 7. Dolerite. 8. Rhyolite (proper). 9. Basalt (proper). * For classification and exact meaning of terms here employed see next chapter. |
