78 APPENDIX TO REPORT OF THE SECRETARY. 



area became the loftier of the two, thus reversing their relative altitudes. 

 The lake area is now a portion of the so-called plateau country, and 

 since the commencement of the miocene (para-miocene) has been subject 

 to a great and continuous erosion. The District of the High Plateaus 

 occupies a portion of a narrow belt separating the plateau country from 

 the basin i^roviuce, and therefore stands upon the locus of the ancient 

 shore-line, which, in the lacustrine stage, bounded the two areas. To 

 that shoreline they stand in an intimate and remarkable relation. To 

 its trend the great displacements maintain, not merely a general paral- 

 lelism, but an approxioiation to strict parallelism both in totality and in 

 detail, which would not have been anticipated, and which cannot be 

 purely accidental, and seems to point to some definite determinative 

 association between the littoral deposits and the great lines of displace- 

 ment. The great structural features are these faults and their equiva- 

 lent monoclinal flexures. They are remarkably persistent, extending in 

 parallel courses throughout the entire length of the region surveyed. 

 One of them, the great Sevier fault, becoming here and there a mono- 

 clinal flexure, has been traced continuously over a length of 240 miles, 

 and others of nearly equal persistence have been noted. The High 

 Plateaus belong to the Plateau country, for notwithstanding the great 

 amount of dynamical energy indicated in their uplifting, they preserve 

 in a remarkable manner the plateau type of structure, which is distin- 

 guished sharply from the arched, flexed, and tilted types prevailing in 

 other disturbed localities. There is an abrupt transition from this 

 plateau structure to that found in the adjoining basin. There are some 

 localities where one may hurl a stone from one province to the other, 

 and in general it may be said that the dividing line must pass within a 

 single range or table. The plateau province seems to stand here in 

 strict correlation to the tertiary beds ; where the tertiaries end there 

 also end the plateaus. The relation between the tertiary and cretaceous 

 throughout this belt is one of general unconformity ; in many places 

 where the contacts are seen, the tertiary is revealed lying across the 

 upturned and eroded edges of the cretaceous, showing clearly a break 

 between those portions of the two series which are here preserved. 



But of all the features displayed by the high plateaus the most re- 

 markable are the manifestations of former volcanic activity. Both in 

 area and thickness the volcanic emanations are very extensive. They 

 cover more than 5,000 square miles, and sections of 4,000 to 5,000 feet 

 are presented without revealing the lowest beds. The greater part of 

 the eruptions took place after the lake basin had been drained or had 

 shrunken to limits outside of the district, for sedimentary beds have not 

 been found intercalated between the various flows, but always underlie 

 them. It is therefore impossible to fix with great precision the com- 

 mencement of the outbreaks; but the general indications are that they 

 began very soon after the close of the lacustrine period, and they may 

 have commenced still earlier. The eruptive epoch was undoubtedly a 



