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Show 68 GEOLOGY OF THE HIGH PLATEAUS. Now, let us make the following arrangement. Place at the head of the series hornblendic propylite. Select from the list in the order given those rocks which are more acid than propylite. Take next those which are more basic than propylite, and write them also in the order in which they occur. We shall then obtain the following grouping: 1. Hornblendic propylite. 3. Hornblendic trachyte. 2. Hornblendic andesite. 5; Sanidin trachyte. 4. Augitic andesite. 6. Liparite. 7. Dolerite. 8. Ehyolite. 9. Basalt. This resolves the lithologic series into two semi-series, each of which displays a distinct and unmistakable progression of chemical and physical properties. The first includes the acid and sub-acid groups, which increase in acidity with the process of the volcanic cycle. The second includes the basic and sub-basic groups, which correlatively decrease in acidity. The law may be thus expressed in terms of chemical properties to which the physical properties stand in a relation of dependence: At the commencement of the volcanic cycle the rocks first erupted are those which belong to the middle of the lithological scale. As the cycle advances, the rocks resolve themselves into two semi-serie's, growing more and more divergent in character, and when the end of the cycle is neared they become extreme in their contrast. Taking Pichthofen's five orders (major groups) and arranging them on the same plan, we may express the same correlation as follows: 1. Propylite. 3. Trachyte. 2. Andesite. 4. Ehyolite. 5. Basalt. Possibly it might be thought that this mode of finding a sequence and a correlation bears a resemblance to some problems in the properties of numbers, in which, any fortuitous collection of numbers being taken and treated to certain manipulations, a law of arrangement appears; the real explanation being a latent petitio principii. But this is not so. Even if we took Pichthofen's five orders only, the probabilities against a merely fortuitous coincidence of orders of eruption with the above double sequence of physical properties would be as 3 to 1. But if we apply the same treat- |