4 MIMNG INDUSTEY. 



tain building, wliich resulted in the Pacific Coast Ranges and the important 

 chains east of the Wahsatch, a final series of disturbances has taken place. 

 But neither are these earliest nor latest dynamical epochs of so considerable 

 geographical importance, nor are thej so closely connected with the objects 

 of this volume, as to merit more than this passing allusion. 



Unfortunately, no general map of the topography of the far west is suffi- 

 ciently accurate in detail to serve as a satisfactory basis for a study of the 

 intricate and extended problem of mountain geology. The Warren map of 

 the United States Engineer Department, by far the fullest and most reliable 

 authority extant, is sufficient to illustrate the larger outlines of topography. 

 A section of this valuable map is given in Atlas-Plate I, including the main 

 central region of the Great Basin, with a part of the coast system of Cali- 

 fornia and the outlying chains of the Rocky Mountains. 



This Exploration has been confined to a belt one hundred miles from 

 north to south, bordering the fortieth parallel, upon which line the mountains 

 reach their widest expansion. It has extended from east to west between the 

 meridians of one hundred and nine and one hundred and twenty, as a con- 

 tinuous survey, and has gone outside this belt to examine certain important 

 mining districts. 



The whole field of labor undertaken by the corps is shown upon Atlas- 

 Plate I ; and all the more productive mining localities, embraced within its 

 observation, are indicated by red lines. 



A brief study of this map will teach the one great and prominent law of 

 arrangement of Cordillera mountain chains, namely, that they trend from north 

 to south, or northw^est to southeast — in other words, longitudinally with the 

 main axis of the whole system of elevation. In strict subordination to this 

 longitudinal direction of ranges are grouped all the structural features of local 

 geology. The average strike of the great areas of upturned strata is gener- 

 ally with the meridian. All the larger outbursts of granitic rocks conform 

 to it as well, since their vents are most commonly the axial lines of actual 

 folds ; and, lastly, when the Tertiary uplift occurred, its ranges bordered the 

 older mountains in parallelism, and the volumes of lavas accompanying it found 

 exit through longitudinal vents, and either built themselves up along the 

 ancient lines of structure, or, through new fissures, piled up chains of volcanoes 

 conforming in trend with the general north and south plan. 



