64 GEOLOGY OF THE HIGH PLATEAUS. 



of the Rocky Mountain Region from the Great Plains to the Pacific (each 

 within the Hmits 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 inci'ease 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 con- 

 spicuous 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 ampli- 

 tude 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 quali- 

 fications which are required in order to express the exact nature of the 

 sequence. These do not essentially aff"ect 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 bo amiss to state here that at the commencemeut of the study I had no prepossession 

 in favor of Richthofen's views — possibly the contrary. I felt rather an intense curiosity. After a year's 

 oxamiuation I was inclined to the belief that his generalization was not applicable to this district, or 

 ■was at most very imperfectly so. It was ap])arent, hov.ever, that there was much complexity, and I 

 determined to examine the best exposures thoroughly and endeavor to unravel this complexity, if possi- 

 ble, in order to ascertain whether any real ord<!r of succession existed, or whether the sequences were 

 only accidental or cai)ricious. The result will be seen iu the text. 



