Xii GEOLOGY OF THE HIGH PLATEAUS. 



should be studied in detail, that the great structure lines, the faults and 

 flexures, should be carefully traced, and the displacements determined quan- 

 titatively; but the most important part of the investigation to be made was 

 presented in the study of the volcanic formations, which are the chief char- 

 acteristics of the group of High Plateaus. No systematic work had been 

 done in this field. Our knowledge of it was chiefly confined to its geo- 

 graphic extent and to a general belief that an extensive series of volcanic 

 rocks would be found, and that the subject was of great complexity. At 

 this stage Capt. C. E. Dutton, of the Ordnance Corps, was induced to under- 

 take the investigation. Three seasons were devoted by him to field labor, 

 and the intervening months were chiefly given to laboratory study of the 

 materials collected in the field. With great labor and skill the work has 

 been accomplished, and its results are presented in this volume, which will 

 be found to extend our knowledge of the geology of the United States and 

 to be an important contribution to geologic philosophy. 



To a large extent the sedimentary region embraced in the survey of 

 which this volume treats is destitute of vegetation and soil and its rocks 

 are so naked that good sections are obtainable on every hand. Again, the 

 region is dissected by deep canons. From both of these reasons the geology 

 is plainly revealed. Every fault, every flexure, the relations of successive 

 strata, unconformities, and all facts of structure are seen at once. But 

 there are two sources of obscurity. First, some of the highest plateaus are 

 covered with forests and vegetation. Second, the extravasated rocks are 

 aggregated in a much more confused manner than the sedimentary beds, 

 and greater labor and care is required in tracing them, and after the utmost 

 care uncertainties and doubts remain. Thus it is that in describing the 

 structural geology of the region the details of examination do not appear as 

 in reports on regions of country less favorable to geologic examination. 

 To a large extent, also, the details of structure are omitted from the text 

 and appear in the graphic illustrations which accompany the report. It 

 has been the policy of the survey to relieve its reports to the utmost extent 

 of burdensome details of verbiage, by presenting them, as far as possible, 

 through graphic methods to the eye. 



The early reconnaissance of the country was in part made by Mr. 



