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Show 64 GEOLOGY OF THE HIGH PLATEAUS. of the Rocky Mountain Region from the Great Plains to the Pacific (each within the limits of his own special field), are almost wholly in accord in the belief that Richthofen's law of succession is there sustained. This great field is indeed not yet fully explored, but a very considerable portion of it has been examined. The display of the phenomena of extinct volcanism is, when taken collectively, probably the most extensive and varied in the world. The magnitude and abundance of the eruptions increase as we proceed westward. In the Basin Ranges hardly one fails to show important masses of eruptive rocks, and in many of them such rocks constitute the greater portion of the visible bulk of the ranges. This is especially true of the southern Basin Ranges south of the thirty-eighth parallel, and still more emphatically true of Oregon, Northern California, and the Territories of Washington and Idaho. Of these individualized areas the District of the High Plateaus is a conspicuous member, though probably far below some of them in magnitude. But among those which have hitherto been brought to notice, none, I believe, present so full and so approximately complete a lithological series. Here then, if anywhere, we ought to find the means of putting Richthofen's law to the test. This was felt after the first season's work had revealed the amplitude and variety of the materials, and throughout the subsequent study of the district was never lost sight of.* As a result of the study, I am satisfied that Richthofen's law is on the whole sustained. Yet there are certain qualifications which are required in order to express the exact nature of the sequence. These do not essentially affect the validity of the law as a whole, but rather are supplementary to it There can be no question that the oldest erupted masses now visible there are propylites. Next in age follow the hornblendic andesites. The third series of eruptions, which were by far the most extensive, included tra-chytic rocks, but not trachytes alone. Their associates will be spoken of * It may not be amiss to state here that at the commencement of the study I had no prepossession in favor of Richthofen's viewsâ€"possibly the contrary. I felt rather an intense curiosity. After a year's examination I was inclined to the belief that his generalization was not applicable to this district, or was at most very imperfectly so. It was apparent, however, that there was much complexity, and I determined to examine the best exposures thoroughly and endeavor to unravel this complexity, if possible, in order to ascertain whether any real order of succession existed, or whether the sequences were only accidental or capricious. The result will be seen in the text. |